Admit something mind blowingly simple that you have managed to overlook since day one. One of those "are you kidding me" moments.
I've got two:
People always talk about how easy it is to make money while leveling crafting jobs. Somehow I managed to net lose about 50,000 gil each time I would level DOH jobs from 1-50. Well one day I decided to sell all of the extra mats/junk I had accumulated and clean out my inventory.... Made back a ton of Gil. Idiot.
Also didn't realize where the "level sync" button was until about four months in. I had always turned off the Duty list on the screen to reduce clutter. When I'd ask, people would say "uhhh...click that big as button that says level sync maybe?"
/facepalm