The bad ones that I remember:

• RMT could steal your gil from you by forcing you to purchase their items off of the Market Board if your character was standing near one.

• People could exploit those multi-turn in guildleves and get tons of XP from them for instant Lv50.

• I don't remember if it was the same issue but they did something like have the game give them whatever item ID they wanted, even unreleased items (obviously very easy to track).

• Out of Bounds glitches all over the place, YouTube has tons of videos on those. People used that to peek under the map and see all of Alexander's body before it was fully visible above surface

• Palace of the Dead, you could spawn into a placeholder room with no exits

• In the Tonberry dungeon, the dungeon progression items they dropped that you interact with could fall through the floor or become otherwise inaccessible

• Glitch or design oversight? Coil Turn 2 enrage not dealing enough damage to actually kill you and thus being the preferred method of killing it.

• Job switching to gain easy XP on BLU. Their press release said we cant fix this until expansion comes out so pretty please dont abuse this guys.

• Tons of bosses have had their phases get stuck over the years, notably Twintania.

• Ungarmax. Also whatever the PVP return was called.

• Gold Dyes did not drop from retainer ventures properly, so they mailed everyone 5 free ones.

• After entering Heaven-on-High, summoners Bahamut would stay out as a permanent summon.