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While cdthirtyfive was questing on his run to 80, I ran across a mob trapped in a staircase. If you got close enough, it would attack and do damage, but, no matter what angle you tried to attack with spells, it would say it was not in the line of sight. If you tried to melee, the mob would evade. Luckily its aggro range was quite small and you could walk on the other side of the steps and fall out of combat rather quickly.