(...)and at least alerted the crew for such a situation and/or
handled it properly by not using the invalid input!!!
But the instruments are not telling lies either (not every single one, at least)... a simple cross check would reveal the odd value,
IF the PM does the
M part.
It would be conspicuous even if all the FO's instruments went bananas, you still can compare your RA with your pressure ALT to see anything strange (never mind seeing a red bar along side the speed tape and the speed arrow trend pointing down)