My usual mistakes happen when I'm doing routine stuff and go on autopilot. Draw a line, cut, debur. Draw a line, cut, debur. Draw a line, cut, debur. Draw a line. (Oh, this song on the radio is kind of nice). Cut. ****, that was the rivet line, not the cut line and now I'm waiting 2 weeks for a new piece from Vans.
Also I make little problems bigger if I keep at them. Oh, that rivet shop head looks marginal, let's drill it out. Now the hole is oblong. OK re-drill for a AD4 rivet. Now edge distance is busted and the part is scrap!