T1 ♠J 4 8 7
T2 ♠3 6 Q 9
T3 ♠K T ♦3 ♠A
T4 ♣A 7 3 2
T5 ♣5 ♥4 ♣K 9
before the next trick is started East says oops! and produces the ♣T and leaves the ♥4 on the table. Declarer tanks for a while and calls director and gets the 4♥ ruled a penalty card and confirms that if East is on lead they will have to play the card but if West is on least declarer can require or forbid the lead of the card.
T6 ♦Q K 9 J
"I forbid hearts"
and a diamond return leads to down 1.
If there is no revoke declarer will run clubs, and then likely plays on diamonds (he can run clubs and lose the hand entry or only play 3 rounds and keep the entry). He will know that east has a stiff or void diamond since he has 55 in the majors and therefore, after showing 2 clubs, will have at most 1 diamond. If it is the K, then down one is made through cashing the A and then the Q. If it is the J then you can make 3 if you run the Q smothering the J. If it is a low spot then playing the Q doesn't cost (you'll be down 2 if west covers), so a good MP line is likely to play 3 or more rounds of clubs and then run the Q and that was my plan. But the 4♥ gave me a way to guarantee no worse than down 1 if east has anything but the K (6-1 favorite) at little cost. Is this something that East could have known would have been damaging when she revoked? Do you adjust to 3nt making?

Help

1♦*-2♦**-3♣
3♥-3♠-X-3nt
all pass
1♦ was announced as could be short
2♦ was explained as Bailey's showing
55 or better in spades and hearts or spades and clubs.