Despite the 4/1 joke I've made a few mistakes along the way, and my students often made mistakes along the way to finishing their design projects.  Personally I'll emphasize the mistaken dado cut or mismeasurement with a contrasting fix.  Not that I couldn't fix such things so no one would notice, but so a more interesting conversation would ensue when the obvious fix was noticed.  Most student "mistakes" ended up as more interesting details/design changes to the final project.  Opportunities to shake a design free from the ordinary.