I think it happens a lot, particularly if the preference for judging is quite strong. I suspect it is because, although we value social considerations and values, we are able to cut through a great deal of extraneous information and focus on the key factor in a given situation. INFJs are natural problem solvers, which many people assume is more of a Thinking characteristic. I think that there are probably very few observable differences between an INFJ and an INTJ. I would guess that, if both were given the same problem, they would likely arrive at similar conclusions. I think the difference is entirely in how they arrived at that conclusion, which is often an entirely internal process, and not something that is obvious to observers.
I am willing to bet that the difference between ENFJ and ENTJ is much more observable, because they are more likely to publicly demonstrate the steps that they take to arrive at a given outcome.