I've written a version of the 1980s game QIX for linux (and it seems to
work on FreeBSD too). Its C++ source and doesn't require any special
graphics libraries, just libX11.so and libXext.so which are standard on
every distribution.
If you kill it by clicking on the window destroy button instead of pressing
'q' for quit you'll see that. Its nothing to worry about, its just the game
trying to access a window that no longer exists.