I have done many stupid things or forgotten things in my career. We once flew Toulouse to Heathrow with the APU running. It was only when the other pilot went to the loo, and I did a casual cockpit scan - an hour+ into the flight - when I noticed the APU was still running ! Ooopps !
Hence checklists.
But they must be taken seriously. A couple of times at my ATPL flight school, I witnessed instructors in piston twins landing with the gear up, which bent the props and all sorts. I now have a personal checklist when on very short finals, when I say under my breath "Fly. By. Wire":
F = Flaps set?
B = Brakes: Auto brake selected and no residual pressure?
W = Wheels down and locked?
The key is to use checklists properly; They are not there to be gotten out of the way as quickly as possible, they are there to check stuff and prevent death. For example, I deliberately do my (Airbus) control checks in a different order to the book, because it is too easy to give the expected vocal response rather than actually looking properly at the flight control page and stating what is seen. More than once I have been holding full right, when the response has been "full left". My response is "Are you sure?".