Say X plays the assassin against Y. Y wants to block it using the contessa, but before he can do so, Z (incorrectly) challenges X. Z loses a card, but thereafter Y also has to discard a card, and is not allowed to play the contessa.
I think this is a bug - Y's ability to play the contessa shouldnt be blocked by someone else challenging X before he (Y) could play it. (but it is also possible that my interpretation of the rules is incorrect). Thoughts?