West Coast,
Perhaps it was fortunate that you did forget, because, as gas-chamber indicates, it's a far too general statement, and will not apply to all aircraft. The 'rule' that you state did apply to one 1950's type that I flew, but not to any subsequent aircraft.
Now, if you're talking about indications of a FAILED AC Synchronized or DC synchronized (dessyn) instrument, that's a different matter altogether.
Happy Holidays,
Old Smokey