Discovered a couple of battle square bugs/quirks (just before Temple of Ancients)
I started off with the compulsory Keystone fight and managed to get through it without issue. I then went after the championship belt, but realised halfway through that I was going to run out of restorative/anti-status items/GP, so decided to restart the game and just do the Keystone fight again. But the second time I did it, I got hugely screwed by the slots - after the first battle, all the slots were break magic materia. Then, after the second, it was break summon materia. And then, it was break weapon.
So naturally I failed. But then when I tried again, everything was back to normal (i.e. every reel returned to having 3 different options). The only thing I did differently was to immediately accept the fight in the case where everything went wrong, so possibly that had something to do with it? Not sure what was intended for that fight.
Secondly, I've discovered that if you use Cross-slash at the start of the Dyne fight, it will break his script and he won't do any of his special attacks (Molotov shot, Heavy shot, Force Armour, Psycho Break, Desperado). I think it must be because it stuns him when he would ordinarily take a specific action, but you would have a better idea of what's actually going on.
Finally, think someone mentioned it already, but the Golems in the Temple of the Ancients are a little broken - selecting an option in their menu always triggers the option above it instead of the one you wanted.