Reposting this from my faq (where I verified this was a bug):

If I play Dreist and the opponent activates Dissidence first, Dreist goes to the discard pile.

If Zahal tries to bring Dreist into play while he has Dissidence attached, it removes itself from the game. This seems bugged to me, shouldn't it play and then send itself to the discard pile (or just stay in the discard pile). I see no reason why it should be removed from the game.

I thought that this might have been due to other cards in play that I didn't notice, but it's happened at least 3 times vs 3 different decks.