Whats the stupidest mistake you've ever made while building?
So far the dumbest thing I've done is being forgetful.
Recently I was working on the wing of a P-40. It looked beautiful and I had already decided to sheet the entire wing with 1/32" balsa. So, I yanked out the balsa cut the pieces out that I needed, formed the sheet around the top of the wing, and glued it down. Then I sheeted the bottom of the wing.
After this I pulled out my sanding block and formed the leading edge and made the trailing edge smooth. Happy with this beautiful piece of work, I attached the wing to the fuse and showed it to my wife. She said, 'It looks great... But shouldn't it have wheels?'
Egads!
I had completely forgotten to build the landing gear into the wing...