The C preprocessor hack for token pasting in K&R C. On all (sufficiently old) preprocessors (invariably named cpp...), saying x/**/y, with x or y macro expansion variables, would yield the concatenation of the tokens. This was documented nowhere, it just happened to be that way. Presumably any comment might work work just as well, but might not; the empty comment had no preferable status. Then again, given the hack's entire undocumentedness, even that wasn't guaranteed.

ANSI C (and therefore C++) introduced a documented way to paste tokens, using the ## operator. Use that, and your life will be a lot less interesting (this is a Good Thing).