I was playing Windows Entertainment Pack Tetris. After the 10th level, it doesn't get any faster (of course it is fast enough) - and so I was playing for a while, when suddenly I noticed that I had a negative score!!! The integer representing my score had overflowed!
The Windows Entertainment Pack version of Tetris must display your score as a signed 16-bit integer (I think it's even a 16 bit program). Because of 2s complement, an integer will after being large positive will be large negative. The number still sorted as larger than the other scores, and counted as a high score, though. Cool.