Bug - Trigger
In the quest "Another crazed mage", there is a random rat outside the door - if it enters before you, as you open the door, it triggers the NPC, not you, resulting in the NPC never talking to you.
I suspect that this can happen with any background NPC + floor triggers.
I've tested this 100% (multiple reloads)
Page 4
http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=478277&page=4Edit - I should have named the NPC, but the quest title is more useful methinks for the content team.