I think bluGill might be referring to cases of undefined behavior which are undefined because the specification literally never mentions the behavior, as oppose

I think bluGill might be referring to cases of undefined behavior which are undefined because the specification literally never mentions the behavior, as opposed to explicitly saying the behavior is undefined. My canonical example of such a case is what happens if you call qsort where the comparison function is "int compare(const void*, const void*) { return 1; }".