Had this happen in Jarsath Wastes on the Danak Shipyard docks.
Needed to talk to an NPC on the docks, but that NPC wasn't there. The quest indicator said she was, and when I clicked on it and stood directly on the indicator it kept telling me she was below me, which led me to assume that she fell through the floor and gave up on the quest.
She was missing for at least a few hours as I logged out and came back before giving up.