I know of a guy that got killed by one stuck shut.

Was a good friend of a guy that worked for me. He lived in an old house where the water heater was open in the bathroom (as was somewhat common in very old houses). He was getting ready to take a shower and the tank exploded (because the safety valve did not work and the tank was old). Scalded him on a high percentage of his body. He was visited by his friends in hospital, was talking to them and seemed to be in good spirits though badly burned. He died three days later of complications and infection. Guy was probably around 25-30 years old at that time which was quite a lot of years ago now.

That is one reason most codes require the hot water heater to be enclosed and not open in a living area. If they do explode you don't want to be around them. The overpressure valve is supposed to prevent it from happening should the thermostat cause an over heat situation and steam and over pressure develops.