I recently learned target painting helps guns hit too, that mwd-ing helps guns hit you too (sig radius), and that mwd-ing your frig against target painting npcs without enough transversal = dead frig.
I also remember learning painfully about how aggression timers work when I was a pvp noob.
Oh, and the day I learned that gcc carries when you leave a system, so it is good to know where you are jumping to. lol.
I also had a bad facepalm when I let my sec status get too low in a low sec system with only high sec exit gates. Was stuck there ratting battlecruiser spawns for days.
Oh, and the time I discovered there are large gangs of pirates near the EVE gate that don't care you're on a pilgrimage.