Not actually a joke but an incident that still makes me chuckle...
Neighbour asked me to lend him a ladder to get at a blocked gutter. I said I'd come round and stand at the bottom to steady it as it was in an awkward position. "No, I'll get the missus to do that..."
I helped him take it round to his house and just parked myself at the base. He climbed up and took the end plate off the guttering, landing me with the inevitable shower... yea, bad planning .
He came down chuckling but stopped and went almost white when I said: "Bet you're glad I didn't let your missus stand there..."