Have you noticed how, in video games, especially the platform kind, enemies don't kill each other? By that I don't mean that they somehow are so clever that they manage to target the player, and avoid the other enemies. No, if they collide, they just bounce back, and if they shoot or throw any flames, only the player gets hurt by it if hit. If any other enemies get in the way, the bullet or flames just pass right through them.

Static HP-tapping obstacles such as spikes or pools of acid act the same way. The player might very well die instantly at the touch of such objects, but I have never in my entire carreer as a console player seen a monster die or even take damage after falling off an edge and onto a floor of spikes.

I can think of at least two plausible explanations for this behaviour:

  • Calculating damage, changing sprites to reflect damage, removing sprites etc. is more CPU- and memory-intensive than mere collision detection. This is not so much an issue with today's powerful systems, but game genre conventions tend to linger for a long time, and the platform genre was started on very slow, weak computers.
  • A naïve implementation of friendly fire among enemies could easily cause most of the enemies to kill each other off, thereby making levels much easier to complete than intended. Programmers would have to extend the enemies' AI to keep them from killing each other, thereby putting a lot of efforth into realism, with little payoff in improved gameplay.

A limited kind of friendly fire does however exist in the game Chip 'n Dale: Rescue Rangers for the NES. Amusingly enough, this only happens between players. C&D RR has a co-operative two-player mode, and if an object thrown by one squirrel hits the other squirrel, the other squirrel will see stars and be immobilized for a few seconds. No health points are taken, though.