I don't know what the worst thing I have done...but here what I have done.
Got the plane together and saw what looked like a bad seam where two parts came together. Well out came the filler and the sand paper and went to work, got that seam to disappear. Then I looked at the plans again and found that it wasn't a seam but an actual panel line. . Tried to rescribed the panel line but I ended up just making a miss of it. It looked okay but the mistake just ruined the project for me.
Joined the fuselage together and while looking over the directions noticed that I forgot one thing....no not the cockpit...but the tail wheel. The tail wheel was to be glued on one side before joining the fuselage. Tried to insert it anyway but seeing the locating hole turned out to be next to impossible. And of course the glue dried so well that pulling the fuselage apart would have destroyed the model. Didn't feel like cutting it open or making a wheels up model so back in the box it went for another try sometime in the future.
Before going to bed I test fitted the cockpit tub, and fuselage fit.. Every thing seemed to line up...went to bed with the cockpit where it was. Next morning glued the fuselage together, let glue dry and came back to move on to the next step. Picked up plane...hear something drop and rattle inside the closed fuselage. Looked at cockpit tub...its gone!! I forgot to glue the cockpit tub into the side wall, now the cockpit sat at the bottom of the fuselage and back from the cockpit opening. While trying to pull the cockpit tub forward and up the joystick snapped off, then the dashboard, so all that was left was the seat and the tub and of course the tub would not move into position, ended up giving up and putting model back in box.
Lastly working on a Me 410, something wasn't going right with something...I don't remember now what it was...got angry and threw the completed cockpit against the wall. Cockpit ends up in several pieces that I never did all of.
All of these happened years ago so I might be better now, but there actually might be more that are more recent but theses all that I seem to remember for now.