Often seen in
anime, this is when someone immediately falls face-first onto the floor with a loud "CLUNK" sound, often as the result of being the
victim of an
atrociously bad
joke.
For example, in episode six of the original Tenchi Muyo OAV, Washuu declares that there is one thing they can do to help in the life-or-death struggle between Tenchi and the villain. Mihoshi asks earnestly what it is. Washu pulls out paper fans and starts cheerleading. Mihoshi facefaults.
A more extreme form of the facepalm reaction.