While this secret has seemingly been removed from the final game, remnants of it still exist – in particular, several one-sided linedefs with no action and method of activation are flagged as hidden. In an interview with the website Doom Depot, level designer Tim Heydelaar claimed that there was an undiscovered secret in Level 20: Breakdown.
The magenta highlights on this map show secret-flagged, tagless one-sided linedefs.
