We'd gone on a family trip to Colorado, and coming back home we stopped at the Denny's in Limon for lunch. The paper placemat got a smattering of my ideas... it's filed away for safe keeping now. 
Well the last two got mine beat.
I once wrote an outline for an article on the back of a (clean) diaper at a softball game.
I've done it in a company meeting before. Those all-employee meetings to discuss quarter results. Look, I'm taking notes, I swear...

I do this for everything I won't remember, which is a lengthy list.I've also been known to leave myself voicemails when I can't find a pen (and I hate texting, so that'll never happen).
in a mental institution. swear to god. and no, i was just visiting--s6
LOL I'm glad I'm not the only one who does this. I'm notorious for calling myself on the way home with lengthy lists, notes, reminders, and other oddities. What's worse is that I usually leave the message on the phone I'm using. hee heeI do this for everything I won't remember, which is a lengthy list.