[AlexMax]

North and east.

It's due to the 'sliding' check on walls, if the player is moving fast enough, the movement path is divided in half and each is checked for obstacles...but if the obstacle happens to be in both segments of the path, you're slid twice.

And the reason the bug only happened when you're moving north and east is due to...another bug, the check is done only if your movement is greater than X or Y, not if it's less than negative X or Y.