I don't really have a "worst" story to tell but I do have a story which definitely lands me on the "How Stupid" list...
I had a bunch of cards I needed to make, all of the same design, so I wanted to get them done ASAP. I finished making 6 cards (assembly line style) before I realized that I'd attached the card fronts to the card base the wrong way! Yep, instead of the card opening like a book, with the card fold being on the left side, the fold was on the right side.
I've never done *that* again! Well, not yet, anyway. LOL