I do a different stupid thing every build. Never the same thing twice, though!
On the E-100, I neglected to paint the insides of the armored exhaust covers, which I didn't notice until the model was on display under the harsh lights at the Shizuoka Hobby Show last year. Whoops! Fixed that as soon as I got it home!
On the BMP-3 I just finished, once I thought it was all done, I sat back to admire my work for a moment. Then I noticed I had yet to remove the liquid masking agent I had applied to the searchlight to keep it nice and clear! After peeling that off and touching up the bezel around the lens, I could finally say it was finished.
Well, I guess one thing I often overlook is the ocassional sink hole, ejector pin mark or parting line.
You think you've gotten them all...everything's absolutely perfect...then right after you apply the final flat coat, you see it: some putty has shrunk a bit and exposed a flaw...
At that late stage in the game, I just accept it. I learned a long time ago that trying to fix something like that only leads to further destruction! Plus, there are always tarps and nets to take care of those problems, right?