I've run into this myself, I -believe- it is due to trying to cast on one of those mobs that looks like it's stuck in the wall. Those mobs aren't actually there, they might be halfway across the zone but due to their Z being off they popup in a wall. If you cast on those mobs you get the problem you are seeing.
I could be completey whacked, but thats what I've been able to figure out.
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VetoEQ
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