This is something I thought about after the latest Main page deletion. You go to a local library, webcafe, or kiosk to do something on Wikipedia and find yourself logged in to somebody else's administrator account.
What are you tempted to do?
What do you actually do?
What I'm tempted to do, delete the administrator's noticeboard with the comment "I'm a moron who forgets to log out from public terminals, please LART me".
What I actually do. Log his stupid ass out, log in as myself and send him a polite and civil email telling him what happened and to be careful next time. If email is not enabled, leave same message on his talk page for all the world to see.