When I tell people I’m a history student, I always get the “oh. History, eh? I hate history” thing. It’s closely followed by the “:dubious: and what do you plan on doing with that?” question.
As a librarian, I have to say, it really is nice. 
But when I tell people I’m a librarian I get, “but you’re a man.”
I get this at work. When I’m doing librarian things. While sitting in the chair that has a big sign that says “Librarian” on it with an arrow pointing at my head.
When I tell people I work in a nursing home they usually say, “Why?”. They think it’s a stop-gap job or that I flunked out of uni. Other nurses are the worst; “it’s not real nursing”. sigh
When people hear I’m a chess player, they immediately ask “how many moves can you see ahead?” 
The answer is about 2 (for each side).
If that sounds small, it’s because chess is difficult and I’m trying to cover every possibility (and there’s a lot of those!).
When people know I am a mother they assume I know everything about pregnancy/childbirth.
Listen. I was pregnant over 17 years ago. I don’t remember most of it and even if I did things have changed so much since then I don’t even know what a “normal” test is anymore.
Oh and when people find out I have nine cats they assume I am a “crazy cat lady”.
My Dad spent his last days in a nursing home and the staff were angels in human form.
Bless you all.
When people hear that I don’t have any kids (and at my advanced age!), they immediately begin to tell me all about their kids and their kids’ kids and the kids of their neighbors, as if the Kid Information Center in my brain must be filled by someone, and they’re taking that task upon themselves for my own good.
Uh, thanks, but I’ve been using that space for storing dusty boxes of old geography snippets just in case I ever get up the nerve to audition for Jeopardy!
When people (guys) hear I work(ed) for a porn site, they immediately say “Can I set up the lights at your next shoot?”
You know, ze French, they have a saying, a translation is like a woman. A faithful one, she is rarely beautiful. And a beautiful one, she is rarely faithful.
For me, when I worked in the securities industry, people had a moment of confusion before I clarified, “no, I’m not a security guard”.
When people hear I work for 911/Police Dispatch they want to hear stories from the job. By stories they mean amusing anecdotes like the woman who called from the burger king drive thru because they wouldn’t give her her meal or whatever. A> That happens very rarely so as to not really be a problem and when it does it can be pretty forgetable. B> The stories that really stick with me tend to be stories you don’t really want to go around telling, lest you scar somebody for life. I’ve collected a handful of stories to put out there that are safe, but this is really pretty annoying.
When people hear I do Improvisational Comedy they ask me to say something funny. (“Wrecked 'em, I nearly killed 'em” is always my response. If you don’t get it, just say it out loud. )
When people hear I’m from Cape Breton, Nova Scotia, they immediately call me a Newfie. 
Then I have to educate them that Nova Scotia and Newfoundland are not the same thing.
When people find out I work for a well-known website (I do internal support, not talking to members of the site), they immediately start asking me for advice on how to handle every problem they’ve ever had on the site. Things like problems with other members, I mean, not technical issues.
I really just answer the “how-to” questions. I have no control over the “why” ones. I really really have no control over or knowledge of why someone else was rude to you on the site.
Damn, I get all this same stuff, and I am only a nursing student! People started showing me their rashes and describing their issues with elimination as soon as I announced my intention to take an anatomy class.
Hey That is good enough for me 
Whenever people hear I’m a graphic/web designer, they immediately say, “Oh, that must be so fun!” Well, I enjoy my job, but it’s not like I wander around with my woodland friends making the world a prettier place. Or, with older relatives, “Oh, on the computer? Must be nice.” Which of course implies I never worked an honest day in my life. sigh
When people hear I’m from Newfoundland, they either:
a) tell me about how they visited Cape Breton or PEI, etc., 15 years ago, and what a nice time they had, a funny reverse on the above poster from Cape Breton
b) say “but you don’t have an accent?? why not?”, sometimes followed by a request to “talk Newfie”. I dislike the term “Newfie” though I am very patriotic (for lack of a better word), and I won’t ask you “whaddyat b’y” on command, because you will inevitably say “what?” with a look of puzzlement on your face.
When people hear I’m in law school they immediately tell me about their first cousin’s daughter’s husband who is a lawyer. That’s wonderful. It has nothing to do with me.
Not trying to pick on you, Emeria, but I think there is a rationale behind this. If someone walks up to me and says they are a student of psychology, I might reply, ‘‘Oh, how wonderful. My husband is in the process of earning his Ph.D. in clinical psychology.’’ This is not necessarily irrelevant information. It communicates that I understand and respect psychology and therefore establishes a bond of commonality between me and whoever I’m trying to get to know. Maybe those people are trying to say, ‘‘Law? Oh, I know someone in law, therefore I understand (on some level) what a lawyer does and wish to express my support and admiration for your chosen career track.’’
When people hear I’m a planETARY scientist, they ask me about the PLANTS in their backyard, and why they’re not growing, or why they’re growing too much, etc. Even here at my university, where our department is often lauded for its involvement in spacecraft missions.
When people hear I’ve worked on space missions, they often say ‘oh so you’re a rocket scientist?’ Not as bad as the plant thing, but no, I’m not a rocket scientist.
When some people hear I’m involved in space science, they ask me about the secret evidence I’ve seen for UFOs. As if I’m part of the them that covers up abductions and visual evidence. I’ve had some pretty intelligent people try to convince me that a certain spacecraft’s first images were of alien cities. I had to tell them that in fact I was there when those images were pulled off of the deep space network, and no, there were no alien cities on them. They still don’t believe me.
When I mention that I had anything to do with nuclear power, they always tell me I must have been terrified of the reactor plant.
Nope. The reactor plant never worried me. The steam plant, on the other hand… (shuddering)
When people hear I’m a cop they immediately tell about every ticket they got that they didn’t deserve.
When people hear I’m a geographer, they say either:
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Oh! My uncle’s a geologist, too!
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Isn’t your job pointless, now that we have GoogleEarth?
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What’s the capital of North Dakota?