Arena of Slakhanan, the epic quest where you fight through multiple waves of enemies for the Heartseeker and Doom Maul gets spawned multiple times, this is especially noticeable on larger maps (I've seen up to 5). However once you go into one, all the other arenas disappears. It's not the most damaging bug, but it does remove the opportunity for other quests to spawn.

Can provide pics/save if needed, but this is extremely easy to reproduce (I've seen it in all my large map games).