This one is probably not politically correct, but then it wouldn't be a joke?
There were three guys sharing a prison cell, (Because of government underfunding, and if in the USA, somebody skimming the Government funding).
They got around to asking each other what they were in for;
The first guy answered; Well, the investment company I was working for was scamming the punters, which meant my commission was being skimmed off. So I decided to see if altering a few figures proved what they were doing.
Then when it did prove the system was set up to defraud the punters, I got done for embezzelment, and locked up so I couldn't 'blow the whistle'. I am in for 12 years. I am innocent.
The second guy answered; Well I am in for forging paintings. I did such a good job on the 'Peastacks', that none of the art critics could tell the difference. So they banged me up for ten years. I am innocent, because the insurance company employed me to do the forgery. Turns out they had a buyer for the original.
Then the two guys asked the third guy what he was in for; Well I got caught murdering my third wife after clubbing her over the head.
So the first two guys asked; Why were you clubbing her over the head?
So He answered, "She wouldn't eat the Mushrooms".
"How long are you in for then"? Asked the other two guys.
"Oh, Seven Years, but with good behaviour I should get off in Five"....