What was the most dangerous programming mistake you have made in C?

Question

I am an intermediate C programmer. If you have made any coding mistake that you came to know later that it was the most hazardous / harmful to the total application please share

Answer 1

I remember two blunders:

1. returning address of an auto variable from within a function in which it was created;
2. copying string to an uninitialised and unallocated pointer to char.