An illegal free or delete is usually an attempt to repeatedly call free() or delete() on an already deallocated block of memory. While not necessarily harmful, this would be indicative of bad design, and would absolutely have to be fixed.

It can also occur when one tries to free a memory block using a pointer which does not point to the beginning of that memory block. This is one of the primary reasons why one should never perform pointer arithmetic on the original pointer one obtains from a call to malloc() or new(), but use a copy instead.