Two people should never see a card played from a different source, even if their plugins don't match. The only case it could possibly happen is if player A moves it to the hand and player B moves it to the discard pile at the same time. Then when player A receives the message from player B, he gets a message telling him the card was moved to the discard pile. And when player B receives the message the message tells him the card goes to the hand. I could implement a system for resolving this conflict, but that is a lot of overhead and it should be a very rare occurrence. It would be difficult to get this sort of out-of-sync error if you were trying to get it.
The error should never happen if you don't mess with the other player's cards, specifically you should let a person move to his hand a card, or move to his discard pile a card.
If you see different cards, that is likely caused by different versions of the plugin. People should be using the same version of the plugin, and there is no excuse not to. You can update a plugin with a single mouse click, you are warned before entering a game with a person using a different version, and when you actually connect to a person with a different version you are again warned that this could cause problems.
