In bridge, is there a penalty if dummy lays down his hand and places a spade with the clubs and it is not noticed by the players until the third round is played?
As a general guideline, all players at the table are jointly responsible for the play and management of Dummy's cards. Consequently, in accordance with Law 64 of The Laws of Contract Bridge (2014):
Law 64 - Procedure after Establishment of a Revoke
D. there is no penalty for the established revoke ...
3. if the revoke was made in failing to play any card faced on, or belonging to a hand faced on, the table, including a card from the dummy's hand, or ....
So although a revoke in Spades by Dummy may have occurred and become established, there is no penalty except as provided in the recap of Law 64:
When any established revoke, including one not subject to penalty, causes damage to the non-offending side insufficiently compensated by the Law, the offending side should transfer additional tricks so as to restore equity.
If no revoke occurred before the discovery of the Spade amongst Dummy's Clubs, then there is no basis for a penalty. However if one of your opponent's is competent at counting to 13, and you are playing for stakes above a couple of cents per point, then I would keep a sharp eye on said opponent in future contests. I have never known a strong player (meaning one who habitually counts as part of playing the game) to not immediately notice any error in Dummy's hand.
As an aside: I am sure that every duplicate club has had a scenario like the following occur every few years:
Pass - Pass - Pass - DIRECTOR!
"This hand has not been shuffled since last week, when I made 8.5 match points with my Right-Hand Opponent's cards for 1NT just in."
That is how well strong players remember all 52 cards of a hand, even long after it is over and scored.
Forget...'s answer is all about a revoke from dummy. That is absolutely correct, but there are other ways that a missorted dummy can affect the play.
The ACBL (bridge authority for North America), have offered the following as best practise for a similar dummy failure: a missing card from dummy:
A director should look at how the play of the hand went and be ready to protect the non-offending side, awarding an adjusted score if necessary. There is also no automatic penalty to the declaring side; a director should also consider how declarer may have played the hand differently had he been able to see all 13 cards. The goal is to restore equity. In gray areas of how the play MIGHT have gone the director should side with the non-offenders.
Other bridge organizations will have their own guidelines for this situation. I expect most to be similar.
I certainly would rule (I am in the ACBL), using that guideline, that when the 7-card diamond suit on dummy was unreachable because we'd seen two full rounds go by (7+6=13), except for the diamond 8 in declarer's hand that matches the "diamond" 8 in dummy, the defence were damaged if they put declarer back in her hand rather than cashing out. Same if partner didn't get their ruff because his "even count" had to be 4 instead of 2 because otherwise there weren't enough spades in the deck.
So, no penalty, as such, but there has been an infraction, and if it caused damage, the opponents will be protected.