The noclip bug can occur whenever the player crosses a tagged linedef (e.g. a teleporter, a trap which opens or closes a door, or a trigger for a lift). Every so often, the action of a tag simply fails. If the linedef is a single-use type (W1), the intended action will never occur unless the level is reset. Wallrunning is one way in which this bug can be triggered, as the player may move fast enough to completely skip clipping checks against the linedefs in question.