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) while moving at a very high speed (such as straferunning, rocket jumping, or through wallrunning). The player moves fast enough that the game engine never detects the line trigger as having been crossed.
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). The bug is more likely to affect orthogonal linedefs; mappers can reduce the chances of the bug happening by using non-orthogonal trigger lines, and doubling important triggers as a precaution.
This should not be confused with a separate bug which causes W1 linedefs to fail activation due to their target sector being busy when crossed. See Single-use line actions can fail for information on that phenomenon.