I've noticed a couple of occasions where I could see an quest marker indicating the location of an NPC, but the NPC was absent. I'm not sure if this is due to the NPC being stuck at some other location or in a different phase or some entirely different bug.
This bug slowed down the Moonhenge quest (tardy NPC was Glarion) and The quest to close the Shadow rifts with Indenir (tardy NPC being Indenir, this happened after the first mini-boss fight where you return to a ritual table). In both instances I eventually relogged and the NPC was then present.