This is one of the things I appreciate about Common LISP's approach: it allows for undefined behavior, but UB is (mostly, as far as I know, I'm sure someone will jump in and correct me ;) ) off by default.

And I think that's the right place for the default to be. C and C++ support UB as well and there are definitely times where you want that for the speed benefits it affords the compiler's output, but getting to UB in those languages is as easy as creating a character array and then copying a string that is too long into that character array (gcc will warn you, but about the conversion of a string literal into a char pointer, not about the buffer overrun).

And with the C++ spec having a page count exceeding the entire Lord of the Rings trilogy, that's a lot of language definition for undefined behavior to live in.