What does a Lubavitcher expect a non Lubavitcherto think if he believes the Rebbe is mistaken about x,(whatever it may be, not important)?
It would be fruitless to expect him to believe that really it’s just perception and really the Rebbe is right etc.
So his options are:
1) A tzaddik gamur can make a mistake.
2) The Rebbe isn’t a tzaddik Gamur (like most tzaddikim out there.)