Yes, that's what happened. I had gone back one move to look at the previous position, and then I think I just went back in my browser history, so that I was looking at the current position on screen, but the server was working with the previous position. Usually I'm careful to avoid such things, but this time I forgot (I must have been distracted by my increasingly uncomfortable position).
I hadn't meant to take back a move, and I've replayed my previous move so that we can restore the correct game history.