What's the silliest mechanical design mistake you've made?

I think this could turn out to be an even bigger 'Mega thread' than perpetual motion... anyway, because its always funny to laugh at other people's misfortune after a grace period, I will. A nameless friend of mine got his first position with a crane building company. They constructed the truly massive ship segment hoisting cranes that move super tanker slices into position. As he was the newbie, he got the Friday evening shift that entailed him sitting on a stop button whilst a hydraulic tank pressure test was cycling. He's a really smart guy, but he really is a walking accident waiting to happen. As he was 22 at the time, the pub held a strong pull for him, in the way that a black hole has for light. What could possibly go wrong? 4 pints later, he came back to a test room knee deep in 50,000 litres of hydraulic fluid.
He had a short association with the Company.

 

Wow, haha good ones guys.

I was designing a prototype for a new product that my company wanted to test, after a few iterations I managed to get to the final version of the product and was accepted to manufacture the functional prototype in polycarbonate which was expensive.

So I logged into my firstcut account and loaded the parts, sent it to quote. The next day I got the email for the quote, I just logged and accepted it WITHOUT CHECKING THE PARTS (huge mistake). After a few days and money spent I received the part and it was missing mayor features, turns out that I saved the part in a rollbackstate when checking a feature and never updated it.

I got all kinds of hell from my boss that week haha. Anyway I always triple check the parts before anything.

 

I can remember only 2 mistakes, one is mine other not. So first my mistake. It was in my first years of use of SolidWorks, I had to order parts from 2mm metall sheet to external company, which accepted only dxf and dwg, so I made my drawing scaled, exported the file and send it. When we received the parts they were earring size and was supposed to be shelf, the guy just gave me the parts and I was looking in my hand and wondering "what the ..." The second mistake was in the last years, a collegue came to me and ask me if I can make manufacture drawings of one crazy bended wire, because firm making it is not accepting 3D files, sure I did the files, but it seems that firm can't read drawings properly also, part was perfect except - mirrored. We were able to use it both ways, but I had to change all visuals and drawings for datasheets also because it was easier than asking the manufacturer to do it proper way.