On my first game I got an even better bug:

  a b c d e f g h

8 ♔       ♛ ♝ ♘

7 ♙ ♙ ♙ ♙

6

5         ♙

4       ♟ ♟     ♕

3   ♟ ♞

2 ♟   ♟     ♟   ♟

1 ♜     ♛ ♚ ♝   ♜

After performing en passant the computer went crazy and the King started taking all the pieces between down row 8. I ended up with a very easy checkmate as I walked my pawn the remaining three rows, unfettered even when it was under double pressure from the night and queen.

I wonder if this is perhaps an issue caused by the fact I used OS X's BSD sed.

Edit: Hmm, can't seem to format it right. Anyways, the bug seems to be present in gnu sed as well. Really the whole game was screwy. I started with e2 e4 and it all went down hill from their for my opponent :).