On linedef 140, there is a texture on the wall with a message violently scribbled on it that reads, "Kill The Shepherd, Save the Sheep." This is a lyrical reference to the song "Magic Fingers" by the band Cursed, which is one of Jon Vail's many favorite bands, on the album Cursed III: Architects of Troubled Sleep.
Similarly, on linedefs 144, 149, and 175, there is a texture on the wall that is high up and most likely cannot be seen clearly without free look, that has a message violently scribbled on it that reads "Digital Dogs with Analog Collars." This is is a song title from the band Trap Them, which is another one of Jon Vail's favorite bands, on the album Sleepwell Deconstructor. The full title is "Day Seven: Digital Dogs With Analog Collars."