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

Question

For example:

char * myString = malloc(sizeof(char)*STRING_BUFFER_SIZE);
free(myString);
free(myString);

Are there any adverse side effects

Answer 1

Bad Things (TM)

Really, I think it's undefined so anything at all including playing "Global Thermonuclear War" with NORAD's mainframe