These bugs do not cause problems today. However they are lurking just waiting to reveal themselves later. The Ariane 5 rocket failure was caused by a float->int conversion error that lay dormant when previous rockets were slower; but the faster Ariane 5 triggered the problem. The Millennium bug is another example of a latent bug that came to light when circumstances changed. Latent bugs are much harder to test using conventional testing techniques and finding them requires someone with foresight to ask.
