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

if (importantvar = importantfunction() == VALID_CODE)

This is when I meant this:

if ((important var = importantfunction()) == VALID_CODE)

This led to many hours of debugging troubles when I assumed it worked like the latter.