I almost hate to mention this but I was cleaning my shop with my dust collector and cyclone only to notice I sucked up a dust bag that was laying next to miter saw. How embarrassing could that be. Come on admit you have one better than this. We all do stupid once in life time.
I was making a nice curio cabinet with glass shelves and glass panes in the door and sides of the cabinet. When I made the final cuts for assembling the door out of oak 1x2's guess what... the door was 3" shorter than what I measured to need. I looked at it and looked at it trying to figure out what I did and realized that the original plan did not call for mitered corners for the door and I mitered them to make them look nicer. Aparently even though I cut everything to length correctly 1.5 + 1.5 = 3 in any language. I ended up making a filler pannel for above the door and may someday switch it out with a stained glass pannel 3" tall x 21" long. Won't intentionally make that mistake again I guess!
I was building a laundry folding table for my wife. I had all the pieces cut, the drawers dovetailed and was ready to glue and screw it together. I had it temporarily assembled to make sure the pieces fit together one last time when my wife walked in and said "looks great, but will you be able to fit it down the stairwell and into the basement?" To which I replied REALLY annoyed, "do you honestly think I would build something that wouldn't fit down the stairs?" She left in a huff.
As I was contemplating sleeping on the couch for my tone with her, I took a minute and thought about it, grabbed the measuring tape and went into the house. The table was 4" too wide for the stairwell. Doh!