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(a == true);
{
  //Do sth when it is true. But it is allways executed.
}

Edit: Another variant of the same mistake.

for(i=0; i