We've all done it, made a really stupid mistake when on a conversion, but will you share it with others so they can avoid doing the same thing? Or are you still in denial?
Cutting a gap for a rooflight was my favourite stupid - long time ago!
I'd got hold of an old telecom van, and decided that what it really needed was a rooflight so I could see when up to no good in the back (I was about 19 at the time...), so I found a rooflight at Melvin Clarke's (the local hold everything hardware store), and having declined to listen to his advice on fitting it (as I knew EVERYTHING in those days) I drew around the skylight and proceeded to cut the whole using a dodgy pair of tin snips I'd liberated from somewhere - without allowing anything at all for overlap. Oops.
When I finally killed said van several months later it still had black bin bag Gaffer taped over the hole in the roof.....