I know of a couple of mistakes in the game's programming, or "bugs" if you want to call them that.

The first is that Marcus' values for statbonus points don't reset to 0 like they should when Eiko joins. This basically means Eiko will start with his statbonus values, and depending on how many levels Marcus gained and what you equipped him with Eiko could start with unusually high stats, and because her level isn't affected by his you can level Marcus as much as you want. In essence she has 196 levels to max her stats instead of 98 like everyone else.

That one was discovered by rebirth flame 2002 at GameFAQs.

-

The second is a Trance bug.

I was trying to figure out the internal mechanics of Trance, and in tests where I set the character's Spirit to less than 12 some very odd things happened. At Lv.1 and 11 Spirit Zidane got 18 turns in Trance. I thought I had messed up while testing because I didn't remember Quina getting that many turns at Lv.1 and his/her Spirit is 11 at that level.

Anyway, turns out that Trance is slightly bugged when dealing with very low Spirit. It works fine for 12 Spirit and up. The only time you'd ever see an example of this bug is if Quina is in the party at Lv.1 and isn't equipped with an item that raises Spirit. From Lv.4 to Lv.6 Quina can get 64 turns in Trance o_o

I kinda discovered this one, but Terence (also from GameFAQs) figured out the specifics behind Trance and confirmed that what I saw was a bug and not caused by anything I had done while testing.