I'm siding a shed today and it remined me of this one;
A contractor hires two guys to put siding up on a garage. He drops them off in the morning then goes to visit another job site.
When he returns later in the day he finds them sitting in the shade doing nothing while the garage sits half finished.
"Hey, why aren't you guys working?" the contract asks.
"We ran out of nails" says one of the guys.
"How'd you run out of nails, I gave you enough to finish the job!"
"The nails were defective, half of them had the head on the wrong side so we had to throw them out."
The contractor looks at the two guys then slaps himself on the forehead. "You morons! Those nails were for the other side of the garage!"