What is one thing you wish you would have known before you retired or separated?

Powerball Numbers

I was a Texas resident, but retired (and remained in) Utah. Texas does not have state income taxes, Utah does - I forgot to change my income withholding, and got an unwelcome $$$ surprise when I filed taxes for the first time as a civilian (again) the following year. I knew, but wish I had been reminded, about state taxes.

How idiotic the civilian world was!