A false step
is a feature of some older houses. The staircase
would be built with the riser
of one of the steps--the false one--an inch or so higher than the rest of them. It functions as a burglar alarm
of sorts: when someone unfamiliar with the staircase attempts to climb it in the dark
--to rob or murder the tenant, for instance--they would snag their foot on the false step, causing them to fall down and make a rather large noise
, waking the sleeping homeowner and alerting him or her to the danger.
Though not a horrible idea, false steps take some getting used to. For the first couple weeks or so after moving into a house boasting such a burglar prevention system, you have to tread lightly and be very careful ascending the staircase, else you wind up with badly bruised shins. It's cheaper than electronic devices, though, and an amusing surprise for house guests you don't much care for.