I have seen this in so many languages and systems for either keywords, table names, and class names. I have yet to see something reserved that was actually implemented though.
Check out the hiostory of the MacBinary [lazerware.com] file format, a way to store Mac Classic files with both data and resource fork, into a flat (=data fork only, compatible with Windows/Unix systems) file. Newer extensions were filling in spaces left open in the first version "for future expansions".
One example... (Score:2)
MacBinary (Score:2)