If you ask me there are 2 different things at hand here.
1) Using ooc info from wherever, forum, irc or so I always avoid as I think it's extremely immoral.
2) Using info because someone sent it to you by accident in game I consider fair play. As if nobles never send messages to the wrong families etc in the real world? Of course messages sometimes went wrong, or got mixed up. Trying to cover it ooc is weak in my opinion, mistake is mistake and you made it IC, so deal with it IC.