I'm still not sure if I have ADHD or not, but it's still a common trait, so I'll say the fact that I keep forgetting everything! Some things, I can remember in meticulous detail, as if it's right in front of me - Calvin and Hobbes strips spring to mind. Other things slide off my brain into the etheral unknown space, and while they're still lodged somewhere in my memory banks, they're painfully out of reach.
I seem to forget a lot of very basic stuff, to the point where it looks like I can't be arsed, and it's as frustrating for me as it probably is for everyone else - if not more so!