In the Paris level, the Mona Lisa painting can be burned with napalm to reveal a cheat code hidden underneath: Up/Down/L1/R1. This four-button code is meant to unlock Cyburbia, a level from the first Twisted Metal game, as one of three secret stages for the 2 Player Challenge Match mode by entering it on the level select screen. However, entering this code does nothing, because the code found in the Paris level has a typo: the Up and Down buttons are accidentally switched, so the correct code is actually: Down/Up/L1/R1.
It was originally intended for Darkside to appear as a stage boss, but the truck was dropped from the game for unknown reasons. According to the first Twisted Metal game, Darkside would have been the first boss in the L.A. level. It would have had a flatbed for what would have been the only time in any of its appearances. It can still be seen in the background of the options menu.