Whats a dumb thing you did that you learned from?
I'll go first--
One of my clients has an office out east and west. The east office is net 60, so when I started working with the west team I billed them at net 60, only to realize much later they were net 30 and they just assumed I wanted to be paid net 60 (why would anyone want that?).
Lesson: when in doubt now I just put "due on receipt".