Gottlieb's Rescue 911 has four white targets on the playfield that serve no purpose. Rumors are that they were meant to spell K-I-R-K for a William Shatner tie-in mode, but it was removed when Shatner's likeliness was not available.
Although Devil's Dare was designed with speech, most machines replaced the original sound/speech board with a sound-only board, leaving the game without any speech or music, and playing slot-machine-style chimes instead.
South Park originally had a "Carpet Munching Race" Video Mode — as Stan, the player competed against Cartman to see who could munch carpet faster and impress Ms. Ellen as the bigger Lesbian. It was disabled soon after the game's release at the insistence of Comedy Central, but ROMs with it re-enabled are available from after-market vendors. Click here for details on the mode.