I agree with Toonia, I think the INFJ is the most mistyped person of all the types. Everytime I hear someone knows one offline, which isn't often, I actively try to meet them, and almost every time, they're not.
Even on the boards, whenever someone says some infj did this or did that, almost always when I find out more, I doubt the person was actually an INFJ. I've read a lot of this here and infpglobalchatter and they often imply they have this controlling or stubborn tendencies, which I highly doubt. I think they're by far the least intellectually arrogant of all the types, by a wide margin.
Most people type by configuring the letters in their head, more F, less E etc, and that's a bad approach. You really have to think about the descriptions, try to look deep into the implications and sketch a complete person mentally, not a cumulation of individual traits or line items. What helps me the most is to know at least 3 or 4 of each type (as well as possible) and try to picture them as the blueprint, even over the literature. When I first got into mbti, I only knew one infj. I spent a lot of time tracking down others just to interact with them enough to understand their type more clearly. I feel the literature is lacking and misleading on analyzing INFJ's in very specific ways - they also mystify the mind reading stuff (which I really think they do have in a sense, but more so with the females). I don't think Keirsey, for instance, understood them fully. There were the hardest type for me to find real life examples of with a close second being the INTJ. I ended up finding 4 of each before I stopped.
I think the majority of the mistyped are actually INTJs, ENFJs, INFPs and ISFJs. One of the difficulties of typing INFJs are that they aren't as outwardly F as the INFP, ENFP or even the ENFJ. They are the least J of all the Js. They are the most T of all the Fs (in my opinion) - which makes typing them even harder. They generally don't let their F affect their judgment. I see them as much of a T as the other T's when it comes to decision making or unbiased assessment of a situation. What makes them F is their desires and motivations, not their execution or analysis.
On top of all that they don't show you what they're thinking, which doesn't help. People expect them to be much sappier and mushier then they really are (they're thinking much darker things than most people realize).