Originally, "bug" designed a problem caused by insects (bugs) present inside the very first computers and that made the "computer" crash. This situation happened really and the name remained. A real "bug" like this one can be seen at the beginning of the movie Brazil.
Nowadays, a bug is the name given to a programming error. As programmers usually say, "It's not a bug, it's an undocumented feature !" (part of the Murphy's law adapted to computers, see external links).
Stunts game engine is based upon vector graphics, but it is known to have not been optimized. Therefore, it happens that some bugs happen during collisions, like the car going up straight to the sky after landing at the border of an elevated road.
Also, it is possible to go through the wall of a cork up/down at slow speed, or you can change car direction and go backward with appropriated trick at the top of a loop.
Collision bugs are numerous in the game, and they are obviously a part of the game interest.
Stunts game engine displays only a few squares in front of the car, and the hills or scenery appears progressively, with a high clipping. but sometimes, it happens that elements don't appear in correct order : you can see what is behind a hill without seeing the hill for example. This graphic bug appears rarely, and only when car is at certain position compared to the concerned track elements.
You can also "force" graphic bugs when doing illusion tracks, check this part of the wiki for more informations.
Another main bug of the game is the ability of some cars to reach an extra speed named powergear in which they don't slow when racing on the grass. Check the powergear page for detailled informations regarding this feature.
Murphy's law adapted to computers : http://www.murphys-laws.com/murphy/murphy-computer.html