There are many INFJs who have tested as INTP before discovering their type (including myself) and INTPs who have tested as INFJ. The two types are very different from a theoretical point of view because they belong to different temperaments (Rational INTP and Idealist INFJ) and have different attitudes towards the world (J and P). The two types share only two cognitive functions and it would seem like there is little reason for indecision between these types. If we base similarity on the common shared functions alone, the two types share only Ti and Fe. It seems more likely that an INFJ would mistype as ISTP (Ti Se Ni Fe) and INTP as ISFJ (Si Fe Ti Ne). But that is not usually the case, INFJ and INTP are more similar than they appear on the surface.
Here are the cognitive functions for these two types:
INFJ: Ni Fe Ti
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