When we moved into our new house we noticed that one of the light switches would sometimes shock you, and sometimes wouldn't turn on at all. When we got someone to look at it they discovered it was badly miswired.

There are two switches that each let you toggle a light on and off, one switch at the top of the stairs and one at the bottom. This should look like this:

Those switches are "three-way," and toggling either switch toggles the light. The diagram above shows a completed circuit and the light would be on, the diagram below shows turning off the light by turning off the switch on the right.

This was all working fine until one of the switches broke. The person doing the work, who apparently had no idea what they were doing, bought an ordinary "two-way" light switch and not seeing what to do with the extra wire connected it to the switch's ground terminal:

This is very wrong, but as drawn above they would have quickly noticed the problem because it's shorting to ground and would trip the circuit breaker. But this is old conduit (BX) wiring without a separate ground conductor, so the ground connection is just through the armored cable jacket. This is an unreliable ground because a poor connection at any joint along the conduit breaks the connection, and that seems to be what happened here:

Which explains why the light would shock people: the faceplate screws connect to the body of the switch, the body of the switch is connected to its the ground terminal, and in this setup the ground terminal has wall current.

This is fixed now, but it does make me worry what else the previous owners did wrong.