It was sometime in the 1970s, around 1976 I think, that there was the crash of an HS-125 near Washington-Dulles Airport, Chantilly, Virginia, when the cause was determined to be the failure of one of the two attitude indicators. Then the pilot flying chose to follow the failed indicator, when I think that the aircraft came apart in flight.
That accident was followed by the requirement that a third attitude indicator be fitted, so that the crew can compare all three, to find that two agree. This is the reason that the standby attitude indicator is referred to as the "tie-breaker," when it is assumed that it shall indicate correctly to agree with the one of the two other systems that is also indicating correctly.