Originally, you could hit the "Back" button and place multiple pawns in the same column in screen chess, for example. I asked Fencer about it, and he fixed it.

About the back button not removing the piece...When you place a piece, it is stored in the server. Hitting the back button simply shows you an older version of the page (in cache memory) without that new piece. Then, when you click on a new square to add a new piece, that other piece is still on the server, so it displays it. Doesn't it remove it when you click on it?