#3 Invisible Birds

[Jim]

I just ran into a strange bug. When 2 flocks of white birds cross each other, they briefly turn black (invisible). Just at that instant, an enemy plane ran into the flock. The result was invisible birds flying everywhere. Just the birds killed me 3 times.

[Simon Howard]

This is an artifact of the fact that XOR is used to draw things to the screen.

I'm having trouble finding an online article to illustrate this, but try Googling for xor blitting.