I found this on a couple of sites but will quote this one:
The Museum of Talking Boards: Ouijastitions If the planchette repeatedly makes a figure eight, it means that an evil spirit is in control of the board.
Crazy...but when we played with the ouija, the movement of the planchette was constantly a figure eight. Of course, we did not know this back then and I didn't even realize this until earlier this year!