One of my smoke alarms has recently been intermittently making the ‘please change my battery’ plipping noise. This morning I took it down, took out the battery, and put another one in. It didn't turn on at all.

A duff battery? Got another one out and tried that. Same thing.

Entire alarm gone dead? Put the old battery back in, and it was fine – did its ‘thank you for activating me’ triple bleep and started flashing its light.

Surely both my fresh batteries can't be dead? I put one back in and had a closer look.

On the old 9V battery, the positive terminal sticks out a little further than on either of the new ones. If I insert one of the new ones and look carefully at it, I can see that the positive contact in the battery socket isn't reaching as far as the terminal! It must have been pushed out of shape by the old battery, and isn't springing back. Gaah!