What if you lose the key?

You know you spend too much time inspecting when you watch a movie (Zodiac) and shout about the deadbolt lock.

In this case, the protagonist, while trying to deduce the identity of the Zodiac killer, visits a home occupied by a guy that qualifies as slightly north of spooky. Hero flees the basement (sensible), heads up the steps, sprints to the door and - Trapped!

He can't leave because the door has a keyed deadlock.

I normally recommend replacing these with a deadbolt that has a knob instead for fire evacuations, but hey, if you live near an ax murderer and need to leave expeditiously, make the change.