>Easy examples of requirements that violate context-freedom are the requirement that variables be declared before use, or the inability to divide an integer literal by 0. (The latter is a compiler error in Go[0] - I'm not sure about C++, but I know the former is a requirement in both languages).

It's usually just a warning, and runtime error. It's actually used in a few cases to attempt to kill a process.

Apple's abort(), for example, uses it: http://www.opensource.apple.com/source/Libc/Libc-262/stdlib/...