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 was dealing with dynamically allocated 2D arrays and I instead of free()'ing N rows, I decided to free M columns. This was fine for smaller inputs where N == M but on large inputs, I only free()'d 50% of what I allocated.

shrug

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