Being medically disqualified for flight. I worked towards the goal of being a military aviator for over a decade, only to find out that my objective depth perception wasn't good enough. I almost went blind on two occasions before then, was one of the first in the country to get a new kind of corrective surgery to get to perfect vision, and never had issues with depth perception (could accurately determine distance out to about 6 miles, +/- about 50 yards, verified on radar). All of that, only to find out that because of the first event that almost blinded me as a child, my brain didn't develop objective depth perception the way it should have. The test where you're given a page and told to pick which circle pops out looks the same to me.

I honestly don't know if I would change anything that I did had I known sooner, because I did still get some positives from my time in the military (along with plenty of health issues), but it definitely would have given me pause.