Overemphasizing the effect of taxes on a trade. I've missed some good trades and made some less than stellar ones, including the single worst of my career, second guessing myself over taxes. Now I just do the best I can and let my CPA worry over the details.
There were so many actually. Early 1990's when large S&P was $500 a point in futures, went long on 3 lot 5 seconds before government report, lost over 10 grand cause huge slippage when I was risking $750 total. I laugh now with all the stupid mistakes, but back then was suicidal most days... Stay in the game long enough, the parts that are the most boring of trading should be learned first, when you become expert level at risk management, one finds that perfect entry not needed.