When you build decks in the web based game, it'll sometimes not distinguish which cards are in your deck and which cards are in your library, so you can actually add the same card twice to a deck. However this will prevent you from playing. Furthermore there's no way of telling whether you have two of a card or one of the images of the card in your library is just an echo of the real card ( but you can add both to the same deck, preventing you from playing).

Also, it takes forever for the web based game to allow you to delete a deck. You have to delete it several times, and reboot the browser once or twice.

THis is using Safari/Firefox 3 on an Intel Mac.