In 2010 a coffee machine ruptured in a supermarket in Hampshire. Luckily nobody was killed, but the picture above does certainly indicate the level of forces involved and the possible results of safety mechanisms failing to operate.
There is an excellent discussion on the potential legal pitfalls of operating a coffee machine in the following document. It discusses who you should employ to test the safety systems on your machine and where the liabilities lie.
Coffee machines should be provided with two safety systems to protect from boiler explosion, in case the normal control system fails.
For a coffee machine with a heat exchanger, the the pressurstat controls the operation of the heater. Should this fail (contacts welded closed, diaphragm split) then the element will remain powered. This fault is protected against by a non-self-resetting thermostat (ie you have to take the cover coffee machine and press a button to reset it) and a separate safety valve to release excess pressure to prevent the boiler rupturing.
Alternatively, two separate safety valves may be used as in the Azkoyen Bravo for example (below).