Even worse is a neutral-earth fault. It may not operate the trip at first if the voltage between neutral and earth is very low at the time. Later, when the local load increases, the trip operates for no obvious reason.
This has a habit of happening at the most inconvenient times, such as just before Sunday lunch when everyone puts their cookers on, or during the night when storage heaters switch in. Because all the neutrals are solidly connected to a bus bar in the consumer unit, isolating the cause of the fault can be a tedious job