Let's look at a few candidates for a short (non-exhaustive) list:
- Bringing in the wooden horse that you have never seen before without checking its innards.
- Invading Russia without cold winter clothing.
- Sam Bowie over Michael Jordan.
- Windows Vista
- Firing Steve Jobs in 1985.
- Not taking out Saddam the first time.
- Pinch-hitting a left-handed starting pitcher with 6 total ABs on the season in the bottom of the ninth inning of a tie game with a runner on second and one out over your slick-fielding shortstop. Oh, and it's an elimination game where one run beats you.