Radiator thermostat failure

I had my first radiator thermostat failure today. Woke up to find the bedroom radiator warm, even though it shouldn't have been. Turned the thermo off and it couldn't stop the water flow. Removed it and checked the valve and that worked fine. Replaced the batteries and it still couldn't stop the flow. It was working, and the stem was being extended but apparently not enough.

Replaced the thermo with a spare unit and that worked as expected.

The system is over three years old now, so not a complaint, just wondered if anyone else had had this sort of failure ?