This has happened twice now ...
Boiler is a Vaillant TurboMax.
Symptoms are that when hot water is running (like last night when I was showering) and there's a pressure drop (when SWMBO uses the cloakroom toilet) something knocks the hot water out. Looking at the boiler, there are no error codes, but the hot water doesn't fire.
Relighting the pilot light using the button on the front seems to fix it.