Tserash is right - Staff 2 (Phase Retreat) is pretty buggy. I was fighting (PvE) centaurs in a town last night, and it pushed me through one of the town walls, so I would imagine that it could also push me through a keep wall. I've also had it push me through one of the invisible barriers around areas you're not supposed to go. I haven't figured out how to control it, though, so I keep winding up stuck in trees and rocks. Using this in WvW would definitely be an exploit, as I understand it.
I've also seen Sword 3 (Illusionary Leap) mentioned as a potential problem child. According to the description, the skill creates a clone at your position, which leaps or charges at your target. However, it's been my experience that the clone actually just appears at the target instead. The secondary function of the skill allows you to swap places with the clone. I haven't tried it, but it might be that you can target an enemy on a keep wall, pop the clone which would appear on the wall next to the enemy (I know for sure this part works with several of the Phantasms, I don't know about clones), then swap places with the clone to put you on the wall.