If you're using tf_halloween_minigame, you have to precache these model:
Code:
models/player/items/taunts/bumpercar/parts/bumpercar.mdl
models/props_halloween/bumpercar_cage.mdl
The first one is required to see any bumper cars. If the second one isn't loaded, the game will crash. Just add two prop_dynamics somewhere (some are using prop_physics, but they aren't necessary and just produce overhead). If you want to hear sounds, you'll have to precache these, too. There are prefabs around that have everything required.