[fen ""]
1. e4 e5
2. Nf3 Nc6
3. Bc4 Nf6
4. Nc3 Bc5
5. d3 h5
6. Bg5 Kf8???
7. Bd5 d6
8. Bc6 c6
causes a bug on "c6". The b7 pawn is supposed to capture the bishop, but instead the c7 pawn just goes on the bishop.
Screenshot
As indicated by Andrew in the comments, try bxc6
instead of c6
. The PGN viewer requires correct PGN, what happens when the PGN is invalid is undefined behaviour.
bxc6
instead, that's the correct pgn notation.b7xc6
andxc6
. Don't know why that didn't occur to me.