Guild Wars 2

Invulnerable enemies: a invulnerability system was added recently so players can’t kill monster when they are on a higher area and the enemy can’t attack back. The problem is that if you attack an enemy, the game will say that it’s invulnerable, and when you go down, it will still be invulnerable.
That commonly happens when parties jump on the wall in AC to break gargoyle statues. It’s impossible to kill spiders from up there, and if you had already attacked one, it will be invulnerable when you go to the ground too.

Hitboxes: when enemies use skills, red circles appear on the ground. But that does not represent the real AoE. Colossus Rumblus is an example: when stones start falling, try staying few steps from the red circle. You’ll still be hit and knocked down.
I believe that happens to player skills as well, like Chocking Gas (Thief’s 4th shortbow skill).