Why exactly should I not call free() on variables not allocated by malloc()?

Question

I read somewhere that it is disastrous to use free to get rid of an object not created by calling malloc, is this true? why?

Answer 1

It would certainly be possible for an implementation of malloc/free to keep a list of the memory blocks thats been allocated and in the case the user tries to free a block that isn't in this list do nothing.

However since the standard says that this isn't a requirement most implementation will treat all pointers coming into free as valid.