Well, we're never supposed to see them. They're never supposed to happen
Using enrico's example, the programmer thinks "hey, if I go into this routine and the data I'm supposed to have doesn't have a length, then the whole kit-and-kaboodle is going up the creek, fast, with no means of propulsion. However, since this should never happen, rather than trying to take it into account with the code, I can say 'hey, buddy, I'm not doing anything with this, it's illogical'" and then just write the normal code relying on it not being possible - either because it will never happen, or because the "assert" will get it if the impossible ever occurs.
Something bad happened. Report it to IBM, they'll fix it.