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

#include 

Me thinking C supports string natively (using Metroworks codewarrior, about 8 years ago).

I did this, for a final project of about 15,000 lines of code. I used this library to do everything relating to strings(appending, splitting, etc.) Only to have the TA's not able to compile any bit of my assignment(using GCC.)

Little did i learn that metroworks had created their own string library. I failed that class.