Here's a situation that came up the other day. I can't remember the cards exactly, but it was something like this
South claims saying "They're all mine".
East, who apparently forgot the spade jack had been played a couple rounds before, says "What about the spade jack?".
South, who had also forgotten about the jack, replies "Ok, then you get one spade trick".
Now dummy steps in and says "The spade jack is long gone. They're all ours".
The question is, can dummy do what he did? And what how many tricks does South legally take, assuming both he and E/W had totally forgotten about the spade jack, and that neither East or West look at each other's cards to find out there is no spade jack after all?
Hope I made myself clear

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South claims all tricks.