Crossed wires
I took the car keys into the garage.
"Here you go" I said. He looked up from behind the counter.
"It's booked into tommorow for the cam belt".
"OK" he said as he cast an eye in his book.
"I can drive it but its parked outside my house... is that OK"
He looked up
"Why can't you drive it?"
"Because I've just had an operation" I said, defensively.
"Oh" he smiled "I though you mean't it wouldn't start.