When Doom was first announced, and the first screenshots started appearing in videogame mags, the fledgeling internet community was abuzz; as cyber-lore would have it, the topic of DOOM was so common on the USENET group comp.sys.ibm.pc.games.action, that someone jokingly suggested that ID's next game have a title less likely to be used as a subject header, and gave the example of, 'Smashing Pumpkins Into Small Piles of Putrid Debris'. or, SPISPOPD for short. People jokingly threw ideas for what the game would be about, and eventually it blossomed into the SPISPOPD FAQ, which later still culminated in the game SPISPOPD, a little puzzler (as I understand it) which was coded in just under 48 hours.
SPISPOPD became so well known that it reached Doom's coders, and one of them snuck in the code - IDSPISPOPD - at the last minute. Sadly, it was later changed to IDCLIP for the sequel, Doom 2. Ah, well. Cest la vie.
'IDSPISPOPD enables no clipping mode. Check it out, you can see the hall of mirrors!'