Some cards, like Mighty Prowess , have the text 'limit 1 per hero.' What will happen if you attempt to play such a limited card beyond its stated limit (i.e., put 2 copies of Mighty Prowess on one hero)?

Per the core set manual, if you play a 'restricted' attachment on a character who already has 2 restricted attachments, you simply discard one of the attachments.

If you try to play a unique card when another copy of that card is already in play, however, the attempt to play the card fails and it remains in its current place.

I don't remember seeing rules for what happens with a 'limit' card. Could be these play like restricted cards, or they could play like uniques. What do you think? I ask because sometimes you want to be able to rid yourself of a 'dead' card, like when you are trying to keep a small hand in the Dunland Trap or Fords of Isen.

I think it is a restriction for attaching a card, so it works like having a matching sphere : if you can't, you can't.

So you can't discard one like restricted, but if you are forced to play it, then it has no valid target and is discarded without effect (not like unique where it should stay where it was).