Once I was coding a pool game, and didn't know why the white ball kept on going on forever. It was 4AM, so I was already a little tired as you can imagine.

The glitch came from the fact that I had designed a whole polynomial function to imitate the deceleration of the ball... I replaced it with two lines: one creating speed and deceleration value, and another one subtracting the deceleration value until speed was zero. And then I went to sleep.