Noclip bug

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 (P1, S1, W1, G1, or D1), the intended action will never occur unless the level is reset.

This anomaly has occasionally been exploited in speedruns as a time-saving tactic (i.e. by recording many abortive demos of the same level until the bug occurs in the desired location).