Stupid travel mistake on my part

I've traveled rather successfully for many years now. I like to think that I know a fair bit more about the travel process despite how ever-changing it is just based on the fact that I have been doing it for a long time.

There was one time though that I made a very costly mistake and once I figured it out I felt so damn stupid about it. It ended up costing me about $1000 as well.

I was in Japan and booked a flight back to the USA, I don't remember exactly what day it was but it was something mid month during off-peak season in an effort to get cheaper tickets, which normally works if you have the patience to wait a few days since at these times the airlines don't always depart several times a day