Comment by mbrock

undefined behavior is the behavior of code patterns "for which this International Standard imposes no requirements" and the behavior is in fact almost always predictable and agreed upon by compiler vendors and the users of the language, which is why you are able to use programs that rely on undefined behavior probably every single second you are using the computer

edit: for example I'm typing this into Safari which means probably every key press and event is going through JSC JIT compiled functions—which have, structurally and necessarily and intentionally, COMPLETELY undefined behavior according to the spec—and yet it miraculously works, perfectly, because the spec doesn't really matter