My favourite example of dealing with undefined behaviour is this:
In practice, many C implementations recognize, for example, #pragma once as a rough equivalent of #include guards — but GCC 1.17, upon finding a #pragma directive, would instead attempt to launch commonly distributed Unix games such as NetHack and Rogue, or start Emacs running a simulation of the Towers of Hanoi.[7]
In practice, many C implementations recognize, for example, #pragma once as a rough equivalent of #include guards — but GCC 1.17, upon finding a #pragma directive, would instead attempt to launch commonly distributed Unix games such as NetHack and Rogue, or start Emacs running a simulation of the Towers of Hanoi.[7]
Source: http://en.wikipedia.org/wiki/Undefined_behavior