I've been noticing this issue for a few weeks now, but it's not a lag issue.
However, I will cast a spell, watch the animation, hear the sound of the spell and nothing happens. Of course my spell is put on timer, and the spell points are taken, but the spell doesn't go off.
I cast it a second time, and it works fine.
It seems like this would be a lag thing, but I'm not experiencing lag anywhere when this happens. It's more like a synchronization issue between the client and the server. The client thinks the spell goes off but the server says "umm no".
So what's up?
Edit - Also as a note, this happens both in and out of combat.