I alt tabbed earlier and a demoman was invisible.
That's a rendering issue, I think. To fix it in-game:
Go into console, type "record namehere", wait for 3 seconds (Everything needs to re-render) and then "stop". All without quotations, of course. And to add onto this, to prevent this occurring every time you alt tab, use windowed mode at max resolution, and boot Tf2 with the launch parameters of "-sw -noborder" (That removes the pesky windows border that's put around the game).
