Well, most of the errors are in the Playstation version. These include:
Aeris: "This guy are sick." (Should be "This guy is sick")
At one point, Elena accuses you of "Doing in" Tseng. This implies he died. In the PC version, this was changed to "Messed up"
One thing that was never -and could never- be fixed: Cait Sith/Reeve use two distinct dialects which don't show through in English. Hence why everyone figures out teh reeve/Cait connection at the same time.
Lots of other little things, too, but i don't know many of them, as i've only played the PC version. Though I think Cait has some bad dialog in Wutai that survived.
Also, Cid swears uncensored in the City of the Ancients, but that's more of a "Whoops, missed that!" than a mistranslation.