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.