I've been INFJ for more than 12 years now, with about 10 years between the first two times I took a test. I have taken about 6 tests, and only once did I score different. I was, at least in that momennt and according to that version of the test, and ENTJ. I didn't feel very different, and a few weeks later again scored INFJ. I do believe that teetering between two letters is fairly common.
I've taken Myers-Briggs quite a few times, and I find that my mood, or the tasks I'm most concerned with tip F/T one way or the other, because I'm borderline on that element. I have no idea how common "borderline" is, or about the standard error of the measurement.
My daughter, whom I always called "Little MH" because she is just like me---I would tell her also that we are "two peas in a pod" (a Southern expression---until a few weeks ago always tested INTJ. I vigorously accused her of being smart enough to "throw" the test so that she was not, in fact, "Little MH." She called me a couple of weeks ago, laughing, and told me that she took it again and that I was right all along: She IS and WAS and INFJ who slanted the results for her own purposes.
What's even worse than #5, is when you deal with an INFJ who is trained in testing methodology. I once frustrated a graduate student doing an eval on me for her training. Not intentionally, but some things just don't always work like you want them to.
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