I've posted this one before, but it's a classic:
A man walks into a bar. As he looks around, he realizes that he is hearing
piano music, but there's no piano player. He goes over to look, and sees
that there is someone playing the piano, but he's only one foot tall.
Thinking this is kind of unusual, the man goes to the bartender and
says "Hey, what's with the foot-tall piano player?". The bartender just
gets a disgusted look, pulls a lamp from under the counter, and says
"Here. Rub this.".
The man takes the lamp, rubs it, and a genie appears. "What's your wish?"
says the genie. "I'd like a million bucks!" answers the man.
Suddenly, a huge flock of ducks flies into the bar and settles around the
"What the hell?" says he. "I didn't ask for a million ducks!". Answers
the bartender, "Do you think I asked for a twelve-inch pianist?".
I know you think you thought you knew what you thought I said,
but I'm not sure you understood what you thought I meant.