Since making that final walk around a habit, nothing. I started doing final everythings, one last check of the radios, one last check of the instruments, etc, etc. It's amazing how many times you pick up on something so obvious that is wrong.
The key is to change your mindset before doing the final check. Acknowledge you are done, go get a tea, and go through the motions.
Somethings I do related to code.
Use `git add -p` to go through changes one by one. Remove stuff that shouldn't be there.
Do a self code review before submitting the code for others to review.
Obvious one, but always run tests.
For what it's worth, I find the same things with maths. I've been working on SLAM systems, so lots of differential geometry and matrix calculus. I'll spend a morning with my notebook trying to figure something out, go to lunch, then come back and go over what I've done.
In university I was horrible at maths exams, because like you I knew everything but made so many stupid mistakes. I think the problem was that during an exam, I could never dial down my mindset from OMG I need to triple check everything right now!