The most likely explanation is that someone's put wires in the wrong terminals, I'm guessing one at a switch meaning you have to turn the light off upstairs, and one in a ceiling rose creating a parallel path making the landing light come on with the bathroom light.
It's possible that something really random has happened like the wires falling out by themselves and falling somewhere else so the circuit works but not as it should, but this is unlikely.
Adam, November 2009
