There was a bug in EQ (among others) back in the day that permitted you to equip items in incorrect slots while, temporarily, duplicating the object you placed. So for instance you could take one weapon you had, duplicate it into both weapon slots, and upon 'removing' it from the incorrect slot, your other weapon would reappear (so you couldn't actually lose anything). This led to some interesting things like people equipping their best gear item (chest, or whatever) to all slots, artificially increasing their power. Best though was with Monks. As this timeframe was toward the later end of Planes of Power, there was an item you could get near end-game, it was an inventory clicker that summoned these throwing stars. The stars themselves were very good stats in general (as was everything else from the end-game zone) but they had a unique trait in that they were VERY high damage:time ratio. They were throwing items so it really didn't matter, but with this bug you could equip them in your primary/secondary weapon slots. With two equipped you could do about as much DPS as another 10 or so DPS classes with zero effort. Lasted about 5 hours so not many got to exploit it.
There was another, also in EQ, where for a day or so Rangers (mixed melee/ranged bow-users/dual wielders) were bugged, as they often were, and their ranged attacks had zero delay. Normally there was a button to press for each 'bow shot' but something got screwy and the button would never truly depress. That led to people building macros that just spammed it 100x on one press. Lotta bosses dropped from solo Rangers that day.