UO- Not 100% sure how it was accomplished, but I showed up one day and my friends had a "flying" boat. The boats z axis was essentially higher then everything, so you could fly over other boats (and houses, thats how they got banned) jump inside and loot, then jump back to your boat. We found one boat with 27 castle deeds on it, traded a couple million gold for a level 50 cleric during vanilla EQ then sold it for 1000$.
WoW - Lots. Tons of bugs on an early Nefarian kill. Rogues could rejoin the fight from running back (gate closed but rogue call ported them back into the room.) OOC pets mitigated the mage call by soaking polymorphs. Well timed divine intervention right after a call allowed OOC ressing.
Best bug though in wow, not sure how it ever went live considering we reported it in the TBC beta. This neck, crafted from jewelcrafting
Citrine Pendant of Golden Healing - Spell - World of Warcraft(pattern started dropping before TBC launch, after 2.0 patch) used your full spellpower value to heal you every 5 seconds. Didn't stack in groups, but you could equip a full set of spellpower gear on say, a warrior, click it, then put on all your regular gear minus the neck and have a huge HoT on you. Made levelling incredibly easy, was fixed without the servers even going down exactly one week after launch. I remember someone in vent saying the neck wasn't working anymore, but the one in our group's was still working. Once the buff expired, clicked it again, no dice. Makes me think someone at blizzard was in on it considering it was up for exactly one week after launch.