My understanding is that MBTI isn't an exact science in itself (I think most would agree), and at its best it only describes tendencies.

People who think mbti is perfect and that any behavior that doesn't fit within a certain framework indicates that this person doesn't belong in XXXX, but in YYYY instead (or any mixture of either).

So let's say the mbti is very nearly perfect. Then let's look into the stack: Say you have an ISTJ. Their fourth function is extraverted intuition. The fourth function is more easily expressed when not functioning like a healthy ISTJ. An ISTJ who has an anxiety disorder or who is going through a rough time at home might come off as an entirely different personality type. They can function in this abnormal state for days and even weeks. Their friends (people who pop up on MBTI forums) might go "is my friend actually an XXXX? They've been behaving this and that way lately".

What kind of replies is this person most likely to get? Very rarely is it "oh, he/she is just an ISTJ in the grip of their inferior function. Help them out!".