What happens when you try to free() already freed memory in c?

Here’s the chapter and verse.

If the argument [to the free function] does not match a pointer earlier returned by the calloc, malloc, or
realloc function, or if the space has been deallocated by a call to free or realloc,
the behavior is undefined. (ISO 9899:1999 – Programming languages — C, Section 7.20.3.2)