Calling free on a pointer twice

Question

I have been taught in lectures, that calling free() on a pointer twice is really, really bad. I know that it is good practice, to set a pointer to NULL

Answer 1

C standard only says that calling free twice on a pointer returned by malloc and its family function invoke undefined behavior. There is no further explanation why it is so.
But, why it is bad is explained here:

Freeing The Same Chunk Twice

To understand what this kind of error might cause, we should remember how the memory manager normally works. Often, it stores the size of the allocated chunk right before the chunk itself in memory. If we freed the memory, this memory chunk might have been allocated again by another malloc() request, and thus this double-free will actually free the wrong memory chunk - causing us to have a dangling pointer somewhere else in our application. Such bugs tend to show themselves much later than the place in the code where they occured. Sometimes we don't see them at all, but they still lurk around, waiting for an opportunity to rear their ugly heads.

Another problem that might occure, is that this double-free will be done after the freed chunk was merged together with neighbouring free chunks to form a larger free chunk, and then the larger chunk was re-allocated. In such a case, when we try to free() our chunk for the 2nd time, we'll actually free only part of the memory chunk that the application is currently using. This will cause even more unexpected problems.