Hello. I hope this may be helpful. It's not scarce information anymore, but I would be happy to keep it somewhere. Thanks.
This thread aims to list everything significant which you could miss permanently and many things you could miss for a long while. At this time, "significance" is assumed for the following: sidequests/their segments/their effects, GFs, missable menu sections (eg, Information entries or Reports), scenes which have falsifiable significance end-game or missable FMVs attached, magazines (most of which are only "missable for a long while"), items which cannot be replaced, subquests, and extremely-valuable-if-technically-replaceable stat-maxing opportunities.
Since this list includes many items which are not strictly "lost-forevers," and some readers may prefer a yet-stricter list, items of irreparable, non-scene-like loss (in the American release and assuming no Chocobo World) are bolded.
Other things which may be missable under some definitions, like the non-unique items in the Prison and many scenes, have been left out for practical reasons of balancing the list's thoroughness and bloat. Cards receive little treatment, but the list covers keeping all cards at least available on D4. If someone wants to make a missable scenes guide, they would be well served to look at this topic's source topic as well as Version One's source topic (follow the chain of links starting with the source link below).
Incidentally, you hazard missing Tutorial/Information entries ("Terms") if you only follow the walkthroughs on GameFAQS.
Editor/Contributor credits: GameFAQs users Arkane Denial, Kyote, Moomba33, psxqwerty9999, _Red___XIII_, shimatani87, The_Kusabi_, Vilurum, xZantetsuken. Thanks. Thanks to the GameFAQs guide writers whom we have referenced as well.
LIST
DISC 11. SCENE: Zell's Love Quest (ZLQ) 1: Speak to the Library girls before the Fire Cavern. There are quite a few scenes in this sidequest, but other than the one you can do as Irvine, none of them need necessarily be done in the suggested timeframe. You could, for example, put them off until after other plot points than those suggested as long as you have enough plot points to move the scenes forward to your satisfaction. They always come in the same order, however (Irvine exception already noted). If you have any questions about these ZLQ entries, see my source: <http://www.gamefaqs.com/boards/197343-final-fantasy-viii/47165843>;. Item-wise, if not story-wise, you only have to do one total to resolve the quest, so rather than listing each first opportunity for a scene as in Ver. 1 as an entry, we will have a [ZLQ] marker to show that before the next missable entry is a potential scene. Refer to Ver. 1 or the above link for more detail.
2. GF: Siren: Draw from Elvoret. You can also drawn her from Tri-Point in the final dungeon. In the Japanese release, no GFs are available in the final dungeon.
3. SCENE: X-ATM092: Whether you decide to have Squall & co. destroy X-ATM092 in battle or to have Quistis destroy it with her giant machine gun is marked and verifiable through the end of Disc 3. If you prefer Quistis to destroy it and see the optional FMV to that effect, this will be marked by the robot's remains remaining on the Dollet beach. A lack of such remains will mark that you destroyed it in battle. The choice is of no necessary significance, but you may now make it purposefully.
4. SCENE: Timber Maniacs 1: There are thirteen of these magazines and an entry from Centra Ruins. The two in Balamb are mutually exclusive, therefore you may pick up twelve TM issues total and get thirteen diary entries after adding Centra. None of the magazines are actual Item menu items. When you pick the magazines up, they disappear, and your having found them is noted in Selphie's Diary (which, being on the Squall's classroom computer, cannot be checked on Disc 4), with one exception forthcoming. There are two magazines in Balamb at this point, and you may pick one of them. Pick the one at the train station and Selphie will write about alcohol; pick the one at the Balamb hotel and Laguna will pass out drunk while visiting Julia but you will get no Diary entry and have fewer total. Basically, both options are but tiny tidbits, both options are invisible in the end, and both options are legitimate steps to finishing this sidequest, so neither is less missable or less perfect. See this topic for an accessible list of all TM entries:
http://www.gamefaqs.com/boards/197343-final-fantasy-viii/50789263 . You may like to keep your own list on this, or leave them be until Disc 3, because I will not direct you to each one individually.
5. REPORT: Battle Report: Talk to Cid at your SeeD promotion before leaving for the dance, or else before leaving for Timber. You can forget the other reports if you miss this.
6. ITEM: Weapons Mon Apr: Check Squall's dresser after the Dollet Mission. Presumably this would stick around until the end of Disc 3. I am noting these magazines not because you could never get another of each in most cases (Occult Fans III and IV are unique, though), but because the game drops them in your lap more or less in sequence, and you many will like to get them as soon as they may. [ZLQ]
7. ITEM: Magical Lamp: Talk to Cid before leaving for Timber. This is the only way to get the GF Diablos without Chocobo World. [ZLQ]
8. ITEM: Pet Pals Vol.2: Get this from Rinoa's bed before leaving the Owls' train. Pet Pals Vol.1 you got automatically, Pet Pals Vol.3-4 you can purchase at Timber's pet shop, and Pet Pals Vol.5-6 I will mention later.
9. TERM: Timber Maniacs: I'm not 100% sure that this is missable, but enter the TM building while in Timber and it won't matter. I imagine you can do this right up until ending Disc 3.
10. ITEM: Girl Next Door: Pick this up in the TM building before Disc 4. This is unique, so you may not want to give it away (perhaps not even when Zone asks for it). If you consider trading it for the Shiva card, consider that Shiva is not missable or even limited-quantity. Girl Next Door is available in Chocobo World.
11. ITEM: Pet Nametag #1 of 2: Return to Timber once you get out and encourage the man outside Timber Maniacs; then come back on Disc 2. (I'll remind you.) You could get more than two if you had Chocobo World.
12. SHENANIGAN: Laguna Pandora 1: When Laguna's lost with the Esthar soldiers: find the first Old Key, loosen the middle lever, loosen the right lever, loosen the left lever, find the second Old Key, hit the red switch, hit the blue switch, push the boulder. See Absolute Steve's walkthrough's section {6} for details.
13. ITEM: Weapons Mon May: Go down the manhole before scaling the Presidential Manor. You can come back for this until the end of Disc 3 by returning to Caraway's house and navigating the sewers again.
14. GF: Carbuncle: Draw from Iguion. You can get it from Krysta in the final dungeon.
DISC 2
15. TERM: Eyes On Me: Ask Kiros about Julia during Laguna's Winhill sequence. It's one of the options while the two of you are reuniting and before going on patrol.
16. REPORT: Character Report: Win this from the card player on Floor 10 of the Prison. I suppose your last chance is when heading upstairs again after Irvine arrives.
17. ITEM: Pet Nametag #2 of 2: Found on Floor 2 of the Prison.
18. ITEM: Combat King 001: Found on Floor 1 of the Prison.
19. SCENE: Friends at the Garden: Bring Rinoa along when choosing the Warn the Garden Team in order to see an extra FMV; you can also hang out with Rinoa (as opposed to no one) before meeting NORG if she's along. If you don't bring Rinoa, but do bring Zell, you can see a short sequence with him instead (this being after the White SeeD Ship has left). If you don't bring either Zell or Rinoa, you hang out with Quistis briefly after the White SeeD bit. Note that, regardless of the above, if you bring Zell to the Library during the Garden Master fracas, the scene and reward will change. [ZLQ: visit without Zell, if possible]
20. GF: Leviathan: Draw from NORG. Can get it from Trauma later.
21. ITEM: Occult Fan III: Look up the Master Fisherman quest AS SOON AS YOU CRASH INTO FH. Split Infinity's section SM#12 is easy to access. Note that the Master Fisherman sidequest has two parts, the former, leading to the unique Occult Fan III, being available immediately and the latter, leading to a Megalixir, being available after the BGH251F2 battle. [ZLQ: visit as Irvine]
22. SCENE: Ward in Trabia Canyon: After the Garden becomes mobile, you have access to the Timber Maniacs magazine in Shumi Village. If you read that before the Laguna movie sequence, Ward won't be in your party during the same. What's the difference? See
http://www.gamefaqs.com/boards/197343-final-fantasy-viii/52309624 .
12. ITEM: Pet Nametag #1 of 2: Go back to Timber and speak with the man whom you encouraged. You may be able to do this as late as Disc 3, but do it on Disc 2 to be safe. [ZLQ]
23. GF: Pandemona: Draw from Fujin. Can get it from Red Giant later. [ZLQ: can now be ended any time]
24. ITEM: Weapons Mon Aug: Found in Trabia. Trabia's open until the end of Disc 3. [ZLQ]
25. GF: Cerberus: Defeat it in Galbadia Garden. Can draw it from Gargantua later.
26. GF: Doomtrain: You cannot get him till later, but you can never get him if you miss Alexander and don't have access to Chocobo World. That means his Term is missable too.
27. GF: Alexander: Draw from Edea at Galbadia Garden. Can get it from Catoblepas later.
DISC 328. ITEM: Combat King 003 and ZLQ Conclusion: You have had chances enough for eight scenes up to this point, and here is a chance for a ninth, in which you should hear that someone is looking for Zell. Regardless, having taken Zell to the Library at least once, go to Balamb and talk to the people near whichever entrance you used. They should have news for Zell. Next see Zell's mom, then stay at the Hotel.
29.TERM: Esthar (1): Talk to Edea at the Orphanage until she mentions Adel; this would generally be part of the mandatory visit you make there, but it's possible to leave without doing it. Adel's name appears in yellow, so it's easy to notice. You must talk to her until you see Adel's name in yellow before punching the wall.
30. TERM: Esthar (2): Same as 'Esthar (1).'
31. TERM: Time Compression: Talk to Cid when you're there learning about the Esthar terms. The timeframe here approaches Disc 4, if I recall correctly from reports of people claiming you needed to do it after visiting Esthar (which you don't, necessarily). There has been some talk about getting this term and 'The Great Hyne' from Esthar, or from Laguna, or from the White SeeD ship. I can't speak to the former two, but I advise you get these from Cid. I do not believe the White SeeD ship has anything to do with it all, though.
32. TERM: The Great Hyne: As 'Time Compression.'
33. ITEM: Timber Maniacs 12: Found on the White SeeD Ship; better get it before leaving for Esthar. Again, these magazines are not proper items, and the only way to verify that you got it is through Selphie's online diary (which can't be done on Disc 4 anyway).
34. ITEM: Weapons Mon 1st: Found during the last Laguna sequence (working in Esthar). See Absolute Steve {18}.
35. ITEM: Occult Fan IV: Pick up after the Esthar Presidential Secretary. See Split Infinity's section SM#19A.
36. ITEM: Combat King 4: Talk to a certain Esthar Soldier before leaving for the Lunar Gate. Talk to a certain Esthar Soldier before crashing into the Lunatic Pandora. See Split Infinity's sections SM#19A and SM#20.
37. CARD: Chances at all, except PuPu: Beat the CC Group sidequest before finishing Disc 3. Doing so will allow you to get any card (except PuPu) as many times as you like (though you may only hold one of each rare card at at time) on Disc 4. Check out section {OPT-8} in Absolute Steve's guide.
38. REPORT: GF Report: Beat the CC Joker at cards. You need to do this sometime between mobilizing Balamb Garden and finishing Disc 3.
39. CARD: Queen of Cards': If you want to complete her quest and get her cards before Disc 4, then do so. See
http://www.gamefaqs.com/boards/197343-final-fantasy-viii/53021144 and/or {OPT-9} in Absolute Steve's guide.
40. CARD: Pupu Card: Finish the PuPu sidequest by giving it 5 Elixirs. Do not lose this card. This is the only absolutely unique card in the game. Getting the card the first time is not missable; you just can't get another.
41. TERM: Shumi Tribe: Ride the Shumis' elevator. You cannot get at the elevator after Disc 3. Why don't you do the Shumi Village Quest ({OPT-5} in Absolute Steve's guide) while you're here? (Because this is its entry too.)
42. SHOP: Any you want available through your menu on Disc 4: Visit them before Disc 4. Absolute Steve {INF-5} has a full list, but his 'Trabia Shop' can only be opened by hacking. Note that some magazines will be missable if you lose access to some of the shops (i.e., Pet Pals Vol.3-6 and any magazine that you did not find).
43. ITEM: Various sidequest items not available on Disc 4: For sidequest stuff not explicitly covered here, see Absolute Steve's sections {OPT-2}, {OPT-4}, {OPT-10}, and parts of {OPT-17}. None of this is unique, except the sidequests themselves. Take a moment here and remember the TM Magazines list I cited at the top of the list.
CHOCOBO WORLD
Power Up 1: There is only one missable within all of the Chocobo World minigame. No other items and Boco's later Power Ups have limited availability. The Event setting must be set at Event Wait On before the first encounter dot is reached after reaching Level 50. If set to Off, this Power Up will be missed permanently. If this first Power Up is missed, then all later Power Ups will be unobtainable as well.~The_Kusabi_
Terms Ribbon and Mini Mog are only available in Chocobo World as are items Friendship and Chocobo's Tag. Certain items which are missables in FF8 are not missable in Chocobo World. Pet Nametags, Magical Lamps, Solomon Rings, Girl Next Door magazines, Hungry Cookpots are notable examples. See Saint's guide,
http://www.gamefaqs.com/ps/197343-final-fantasy-viii/faqs/6615 .