This is something I struggle with, though a few bad or non-wonderful experiences are teaching me to look out for it more.
I am sure that anyone can fall prey to it but this may be a particularly INFJ blind spot. For me, it is a combination of a) being a person who pretty much means what they say, most of the time; b) giving people too much credit for potential, rather than what they're actually doing here and now; c) seeing/hearing what I want to see/hear, particularly if I'm somehow emotionally invested (either with a close friendship, or romantically); d) perhaps according too much of a sacred status to words...?
I've fallen into the trap before, in various situations. Someone runs down a certain social group...but still spends their time hanging out with them; I trust them to figure it out eventually, even if this has been the case for ages. Some guy tells me how special I am to him and how I'm the one person who has stuck by him, who he can really talk to, etc etc...but doesn't ask me out or really make that much effort to spend time with me; I think he's just hesitant and will get there eventually. Needless to say, these types of situations have let me down, big-time.
Have you had the same problem - especially INFJs, NFs, or anyone who wants to weigh in? What have you learned? How do you avoid falling into the trap? Do you think most people are kind of like this, or do you just learn to keep more distance with the ones who behave this way?