Raymond Chen has a series of blog posts (e.g. https://blogs.msdn.microsoft.com/oldnewthing/20140627-00/?p=...) that cover some surprising consequences that undefined behavior can have on program optimization. I'm kind of sympathetic to the point of view that these are abuses of the freedoms granted to the compiler, and that it would be better for it to just generate instructions that match the illegal code. But as others have pointed out, you can generally get the compiler to warn you when it detects something like this, and anyone who uses C and doesn't turn on "treat warnings as errors" gets what they deserve. If you want a language that does runtime bounds checking there are plenty to choose from.

See also this related SO question, and ones it links to: http://stackoverflow.com/questions/24296571/why-does-this-lo...