This is based basically on sightings on INTPc. According to this hypotesis, the "defining" traits of an INTP, such as highly "analytical", "logical", even "intellectually superior" in worst cases, may very well mislead an average ISTJ, who encounters the MBTI for the first time. Because it is not uncommon for an ISTJ to take pride in his "rational", "logical" thinking, the misfortunate ISTJ will assume that indeed he must be the INTP - the most logical and rational type.
At the first sight, he may appear to be one - analyzing and rationalizing everything like the world's biggest scientist, but there are definite deviations from INTP archetype once observed closely.
They are among us.
What do you think? I am not claiming anything, that's why I call it hypotesis. (Also, I do not claim that the percentage of ISTJs mistyped in this way is disproportionally big.)