I have an odd problem. A light is on 3-way switches at each end of the hallway. Sometimes you can switch the switches and the light will not come on. I changed both switches, and it made no difference. They are wired correctly. Sometimes the light will switch on. When it is on, either switch will turn it off. When it is off, sometimes it will go on by itself. Obviously one switch is in the on position at the time. Sometimes we'll go to bed and the light is off, then wake up in the middle of the night and it is on.
I could understand this a bit if something happened to jiggle wires or something, and a loose connection was being jarred on or off. But no. Nothing seems to happen to change their on/off state.
It's a bit of a mystery.