Apparently Mediocre is having some trouble playing Machine vs. Machine games in WinBoard. I had not noticed this until it was brought to my attention, since I use Arena to play those games.
It seems to work if Mediocre is the first engine, but not if it is the second, or if Mediocre is playing both sides.
My best guess is this has something to do with the 'force' command which is not implemented in Mediocre yet.
I will keep a close watch on this, and hopefully it goes away once 'force' is implemented.