Recently a dippy about to leave (so not really bothered) left my immersion heater on for over 24 hours. I only knew anything about it as I was leaving the house, noticed the toilet cistern wat hot (boiling!) checked the cold taps (upstairs) and they were boiling too.
I suddenly remembered the tragic story of the wee girl scalded to death in her cot when the cold tank got hot and melted and the water came through the ceiling onto her. This was caused by a faulty thermostat on the immersion heater.
I quickly checked immersion override switch (which we have never used as oil boiler heats plenty of hot water twice a day) which had beenswitched on, I flushed the loos and ran all the cold taps. Went into the loft, cold tank was bulging out at the sides and turned off the tap to refil the cold tank.
The tank had started to melt, just a tiny hole, but it was on it's way.
It has cost me just under £1,00 to put it right, but if I hadn't noticed it could have been far more serious .
Just a cautionary tale (I sound like the Daily Mail )