I haven't been able to determine the exact trigger for this, but it repeatedly happened to me when I was trying to pull Salrand in Borean Tundra for a quest (she wouldn't pull, kept saying out of LoS and using DnD/Blood Boil directly on her wouldn't pull either), and it happened twice so far when I was farming Netherweave in Shadowmoon Valley. Here's a clip:
It's probably happened to me a total of 8 times ever.
[Edit] I see in this one it says my placement wasn't in LoS. Maybe that's it, but it shouldn't cast then, right? In the example above with Salrand, there isn't anything visible that would put her out of LoS.