All of you (any and all) what is retrospect is the biggest mistake you have ever made.

Selling my laptop from a sketchy overseas ebay purchase. I lost $1000 that day.

Taking a year off after High School before starting college, and passing up on scholarship offers in the process.  That or standing too close to the plate when I was 9 or 10 playing baseball.  I was catching and it was just a pick up game, so I wasn't wearing a mask.  Batter, for some reason took an odd step as they swung, and I got a face (well, forehead) full of bat on the back swing.  Cracked skull, a bunch of stitches, and I still have the scar to this day.  Good times.