"You know what thought did? He didn't do anything - he just thought he did."
A piece of nonsense which basically points out the futility of thinking that something has been done without actually going and assuring oneself that it has in fact been done. So for example
"The basement's flooded!"
"But I thought I turned the faucet off..."
"Well, you know what thought did."
He buried a feather and thought it would grow a chicken
He pi***d in bed and thought he was sweating.
He pi***d on his head and thought it was raining.
He followed a muck-cart and thought it was a wedding