Here's one that just possibly lost me the game:
If you cast
Tainted Pact while you have a
Bolas's Citadel on the battlefield (or any similar effect), each time you exile the top card of your library, you're supposed to be able to see the new top card of the library, before you actually choose whether to put the exiled card in hand or not. The game shows only what was the top card when you first cast
Tainted Pact. I suspect this is an issue with effects that let you see the top card of your library in general.
Bonus stuff:
Resolving
Tainted Pact is a horrible mess. It would make things much easier if you could actually see the card(s) that you exiled. The automatic confirm thing doesn't actually do anything with
Tainted Pact, it would be really nice if you could right click on the "No" button and automatically say no to everything, thus exiling your entire library, but I think the game sees each different card name as a different text/ability. Also, the text which pops up while resolving it is missing a space before the cardname.