Found it!
Duh.
I had made the "setGame" method in player clever - when you set the game, it also automatically added itself to the game. However, this method is also used by the toplink engine to set the game attribute of the player, and it was getting itself terribly confused because (of course) the list of players in the game object is actually a weird clever list that does stuff when you add and remove things.
Ok. Now I can proceed with the rest of this masterpiece.
[Message sent by forum member 'pmurray_bigpond' (pmurray_bigpond)]
http://forums.java.net/jive/thread.jspa?messageID=261421