My roommate has a mid-90s Civic (eww) and has one of those levers or whatever inside that pops the door. She forgets about it and has to open the door again about 50% of the time. Except somehow the cable snapped so she pried open the door and now just leaves it that way. The car's in poor shape, it fits with the theme.
But anyway, I would totally forget about it ALL THE TIME if my Focus had a locking cap, Id go insane sooner rather than later.