Besides undefined behaviour, there is also the equally nasty implementation-defined behaviour.
Undefined behaviour occurs when a program does something the result of which is not specified by the standard.
Implementation-defined behaviour is an action by a program the result of which is not defined by the standard, but which the implementation is required to document. An example is "Multibyte character literals", from Stack Overflow question Is there a C compiler that fails to compile this?.
Implementation-defined behaviour only bites you when you start porting (but upgrading to new version of compiler is also porting!)
