The culture thing that MMM said is huge! I think there is no way of being fluent in Japanese without understanding the culture. Sometimes you run into Japanese people who don't seem to have a firm grasp on their own culture and they are pretty much grouped with hennagaijin.
I admit that I thought "romaji" was "romanji" for the first two or three years of studying Japanese. I feel like my first couple of teachers said it "romanji". Maybe it's referred to as "romanji" in a book somewhere?
"romanji" - Google Search
1,210,000 results for "romanji"
"romaji" - Google Search
5,970,000 results for "romaji"
Romaji is being mistaken for 1/5 of the time (at least according to google)!
Romanization of Japanese - Wikipedia, the free encyclopedia
"romanji" also comes up on wikipedia (and the article says that "romanji" is indeed wrong)