I have a small ritual that I perform every morning when I leave home. I lock the front door and then turn the knob twice to make sure it locked. Later in the day, if I suddenly think, "Hey, did I lock the door?" I can flash upon the knob turning and remember that yes, I locked the door.
This morning I was carrying a bunch of stuff when I left home. My jacket, some AutoCAD manuals, a DVD for a co-worker. My hands were full. When I got to work I could not remember turning the door knob. I was reasonably certain that I had, but try as I might, I could not bring up the picture of my hand trying the door. This is one of the few sorts of things that actually bothers me. Zen as I am, something like that will worry me all day, particularly when I am keeping Trish's cats because then I feel I have more responsibility. If some footpad were to try my door and find it unlocked, not only might they steal my stuff, but they might leave the door open and Bruce and Amelia might become lost.
So at 9:30, once commuter traffic has died down, I took an early lunch and drove home to check the door. It was locked of course. My co-workers found this vastly amusing, but I should point out that none of them live alone and therefore don't really have the same situation. There's always someone they can call if they think they left the stove on or the door unlocked. I could have called Bruce I guess, but he's not too good with doors and I don't really want him learning how the lock works.