I'll make a start...
My first one is misspelling "entrepreneur" on top of 8 pull-up banners for a charity. It was probably 200pt text or something. They had to be reprinted. I guess that's less of a design mistake and just a typo – a big typo, but a typo.
My other one I can think of is setting each panel of a tri-fold leaflet to exactly the same size.
I now try to create less mistakes and help other people create less mistakes with The Print Handbook.
I'd love to hear about your mistakes so that I can try and prevent people making the same ones in the future. Thanks. Can't wait to hear what you have to say.