Bug: If a foe changes form mid-fight (I'm looking at you, Bahamut) while I have a subwindow open (spells, etc.), the ATB timer keeps going even if I have the timer set to wait. This is a minor bug, and reentering a submenu fixes it.
This seems to me like a bug: The negative statuses of the Cursed Shield equipped on the party ignore status effect immunity like from Ribbons! Vanilla FFVI allowed status effect immunity to override this curse. (Is it still uncursed by grinding 256 AP while equipped by someone who isn't Gogo/Umaro?)
Seeming Bug: The Blue spell Death Sentence says it ignores death immunity, but it hasn't worked on bosses when I've tried it.
Oddity: Phoenix Downs auto-kill Undead, even bosses, in the early game, but work inconsistently on enemies throughout the game. Dullahan is clearly Undead, but immune to Phoenix Downs, even though his Wisp helper is affected by Phoenix Downs.
(I'm glad I read your instructions on how to get character-specific costumes from mastery. Otherwise, I would have called 'bug!' Crisis averted! Alleluia!)
Thankee!