Sometimes, when you read too many language standards and various specifications, you start using these weird words. Undefined Behavior is the ultimate evil, a raptor that storms in when you dereference NULL pointer, use freed memory, or violate any other preconditions, which are placed in every dark corner just to keep you awake. Futile attempts to make jokes aside, it's the wording that shows that compiler, library or system can guarantee nothing if you do that.
In most cases, nothing serious happens. Under the most common violations, such as dereferencing memory pointed to by an invalid pointer, you just get "Segmentation fault" (or "Access Violation" if you use an alternative OS) fatal error.
The Raptor Exists
A couple of days ago, I was trying to get wi-fi working at work and downloaded drivers from vendor's website. They happened to belong to the other kernel version, but I installed them, hoping that Undefined Behavior won't bite me much. When I rebooted, after the attempt to do network startup, something undefined broke there. And I observed the following picture...
The raptor was right here, already chewing my hand and smiling with an anime smiley ^_^.
So, guys, if you still don't believe in these tales about raptors that crush you if you do something unwanted, you'd better start believing. You have been warned.