I've been mistaken for Russian by Russian immigrants here, although I am 1/4 Russian (Belorussian to be specific) and another 1/4 Polish by ancestry. A few people have said it's not just the Slavic facial features, but that I have the gait and countenance of a Russian. It's never happened to anyone else in my family, even the ones with more Slavic ancestry. Russian culture is more introverted than American, and smiling in public isn't normal-- so maybe its a combination of my shy and socially reticent nature combined with the 50% Slavic ancestry.
One time a walking on the street smoking a cigarette some started talking to me in Russian and I just looked at him puzzled; then in broken, heavily accented English he told me he needed a light and then said: "You do not speak Russian?! You are not Russian? How can you not be Russian?! You loooook so much Russian!" Then another time in a college class I arrived about 10 minutes early on the first day sat next to a girl who was speaking Russian to her friend seated on her other side, after class she approached me and asked if I was Russian, she said she thought I was listening in on their conversation, just pretending to be a non Russian-speaking American.
Funny thing is, at the time I didn't know much about Russia or have much interest in it, but since then, Russia has become one of my special interests/obsessions, and now I've learned to speak Russian pretty well. I don't speak it well enough to pass as native, but well enough that many Russians say they think I must be a native speaker of some other Slavic language.
I was also once thought to be Hispanic-- considering I'm tall, pale, blue eyed and light brown haired I dunno how that works.
A few times I've been thought Jewish because my Slavic surname looks Jewish to many Americans, and a few times because I knew so much about Jewish topics and can even speak a bit of Hebrew (the history of the Jewish people is a special interest of mine). OTOH another time, at a Jewish acquaintance's wedding, where the attendees were probably 90% Jewish or so, I was asked, "you're not Jewish are you?" by a woman there. I asked how she knew and she said I "didn't look Jewish."
Not an ethnicity, though often carrying certain ethnic connotations-- but some people have also assumed I must be Catholic when I revealed an immense knowledge of the history, practice and ecclesiastic procedures of the Roman Catholic Church (another special interest).
Once I someone said I look Italian, no idea why.
Once an English person said he was shocked to find I'm less than 1/4 English by ancestry since I looked "so English."
Several times I've been told I look like I'm a European (from Europe) though I'm born and bred in America. One time it was because I was smoking a pipe, which is not common for twenty-somethings in America (though I don't believe it really is in Europe either). Another time at a party someone told me they thought I looked like I was from Europe-- I think it may have been because I was being such a wallflower that they thought I didn't speak English well. Another time I was eating a croissant and drinking coffee while smoking my pipe on my college campus. Someone approached me and said they thought I was European because the pipe-coffee-croissant combo and "the way" I was eating/drinking seemed "so European."
When I actually was in Europe (Austria) for two weeks, people never seemed to think I was American 'til I opened my mouth and revealed poor German language skills. My family joked that I would be easily picked out as American since I didn't try to wear "more European" clothes and even kept wearing my Baltimore Orioles cap, but the opposite. Several acquaintances also said they tried to dress "more European" when in Europe so as not to be too conspicuously American, yet were still usually picked out as American tourists very easily.
So maybe one's introverted nature can make one seem less conspicuously foreign when in a country with a more introverted culture; and the same introverted nature combined with one's personal eccentricities can make one seem a foreigner in his own country.