I don't know if there has been a thread about this recently, but this is a subject I've been thinking about.
It seems a number of ISTPs I know think they are INTPs, test as INTPs, and/or are perceived as INTPs (doesn't seem the reverse happens as often). This is a distinct type of ISTP, who doesn't fit stereotypical type descriptions of the ISTP as the fearless, adventurous, "cool" person. This type of ISTP is less active physically and more active mentally (although still more physically active than INTP). Many times they are interested in the same topics as INTPs. They are interested in theory, but unlike the INTP tend to be economical in the time they spend on theory and are more practical in their use of it. Also, ISTPs tend to use some Ni, which is more noticeable when you get to know them better. And the ISTP tends to be less talkative than the INTP.
It appears since the dom function of both types are Ti, when either relies heavily on Ti and/or doesn't have as well-developed an extraverted function, they can look quite similar. It's like their secondary function only exists to focus their Ti in certain ways.
Has anyone else noticed this (particularly INTPs and ISTPs)? In what other ways are the two different?