[Xplosive]

Maybe if their very introverted or they are discouraged using Ne, i.e. secondary Ne is underdeveloped compared to tertiary Si.

[Mutatio NOmenis]

How the hell did you get INTP and ISTJ mixed up?

[Iapetus]

Is your friend fairly tall? Y or N (over 6 ft.)
Is your friend quite creative? Y or N
Are your friend movements jerky or smooth?
Is your friend athletic?
Does your friend speak with a monotone voice or an animated voice?

I ask these questions because it gives information that is needed to flesh out a real person.

[Functianalyst]

Frannyy, intellect + common-sense = wisdom, and you have it in droves. To add to this thread, it's not that impossible for an INTP to think they're ISTJ for the same reasons that ISTPs can mistake themselves as INTJs. Both ITPs (only my opinion) that have strong introverted attitudes can over identify with their tertiary function (INTP=Si and ISTP=Ni). Also let's not discount that we do use all four temperaments in some fashion. If the INTP's temperament preferences are NT-SJ, then it would be quite easy for the confusion.