I can see this thread turning into a oneupsmanship challenge:
Back during car fabrication (see sig) I was making several critical suspension parts. As they were made, they were set off to the side, and at some point I got distracted. Anyway, looked around for some cut-off scrap material to use for the last part, made it, and then noticed even though I'd made four of them, there were only three there. Yup... I used a finished part as the raw material to make the last one, and machined it from the other end, so I didn't notice. Sigh...