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

This is a famous historical example (not something I did), but

double d;      // d gets populated with a large number from somewhere
short s = d ;  // overflow

led to the explosion and total loss of an Ariane V rocket.