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 take the definition of dangerous as "we may ship with that bug and discover only years later when it's to late":

char* c = malloc(...);
.
.
.
free(c);  
.
.
.
c[...] = ...; 

or

// char* s is an input string
char* c = malloc(strlen(s));
strcpy(c, s);

But if you write multiplatform (not limited to x86/x64) this is also great:

char* c = ...;
int i = *((int*)c); // <-- alignment fault

And if your buffer comes from an untrusted source.. basically most code around is dangerous.

But, anyway, in C it's so easy to shoot yourself in the foot, that a topic about shot feet could go around the thousands of pages.