For example…
All women between the ages of 25 and 35 who live in upstate New York will list Ani DiFranco as one of their favorite musical artists.
Everyone who was born in the 80s will have a favorite Ninja Turtle.
For everyone under the age of 10 when The Transformers: The Movie was released in 1986, the question will not be “Did you cry when Optimus Prime died?” but “How much did you cry when Optimus Prime died?”
So what can we assume about your fandom?
That all Thundercats fans are furries
That all furries are perverts (it’s not about sex, dammit! It’s about liking a certain genre that just happens to also attract a bunch of really screwed up people)
That all anime fans are super-geeks who never leave the house and wish they were Japanese.
That everyone who likes to wear kimono is a) Japanese (nope), and b) a stick (I am, but I’ve seen a lot of curvaceous women who look awesome in kimono)
And… I can’t think of one for Batman or Quantum Leap.
That every single female Def Leppard fan is a raving, slavering lunatic who has no life, weighs 600 lbs, lives in a house full of cats and is convinced one of the band members is going to divorce his wife and marry her.
As a mixed martial arts fan, it’s assumed I’m an out-of-shape internet tough guy who in real life couldn’t fight his way out of a wet paper bag, but acts as if he invented jiu-jitsu.
As a heavy metal fan, it’s assumed I’m a misogynistic, homophobic meathead who’s incapable of appreciating subtlety or intelligence in music.
As a fan and owner of a Chevy Corvair, it is often assumed that I blame Ralph Nader for its demise and think it really was an excellent car. (Nope and nope.)
Science fiction fans wear Star Trek costumes to their conventions.
I love Firefly, it’s assumed wrongly that I like Buffy: the Vampire Slayer also.
…which then leads to a SLEW of corollary assumptions. 
As a hat 'n boot wearing country fan, it is regularly assumed I am a right-wing, homophobic, racist, flag waving nutjob.
In fact, an African American friend in a bar won a drink (and a phone number) off a girl who (not knowing he worked with me) pointed to me at the other end of the bar and ascribed said beliefs to me. He bet her that he could walk up to me and get a hug in the middle of the bar. Silly girl took the bet.
This is more a false assumption about what I’m a fan of. You see, I speak fluent Japanese (or at least I used to) and I like science fiction. This does not mean I like anime or Pokemon. Yes, I can translate them. No, I don’t want to, unless you pay me money. No, I really don’t like anime, nor have I ever, even when I was living in Japan. Now leave me alone with my Arthur C. Clarke.
Well, to be fair… I did. Won an award, too.
My love of the comic book/strip medium automatically means that I care, or have ever even heard of, your favorite superhero/villain/other.
Yankee Fans:
It is often assumed we are heartless & souless or that we just jumped on the bandwagon.
I’m another one of them there fans of Firefly who don’t particularly like Buffy.
Also, people give me funny looks because I’m a non-sensitive non-New Age guy who likes Tori Amos. What can I say? She writes compelling stuff.
As a fan of the LOTR movies, I apparently have no life (the fact that I have no life is completely incidental) and also haven’t read the books at all.
Assumptions… FACTS… it’s all the same. 
As a Star Wars fan, I must hate Trek fans. And vice versa. Which would mean I would have to hate myself twice, when in reality it’s barely once!
Well, if it’s an aspect of my musical fandom (Tori A, Kate B, Sarah MacL, Enya, etc.)- it’s that I’m a New Age/neo-Pagan politically leftist Bi-Les girl.
I don’t know if this counts, but…
I’m a fan of sleeping with both sexes. It is mostly assumed that I’m a gay boy who can’t admit it. And if they do, it’s usually assumed I’m a slut. (Which I’m not, btw)
As a SCAdian - that we all want to be Vikings or secretly want to decorate our homes like castles complete with no electricity and rushes on the floor. No thanks, I like playing dressup once a month, and that’s about it.
As a Goth/postpunk fan - we’re all skinny white folk who wear black and mope. Dude, you have no idea how silly I look with eyeliner, never mind greasepaint.