More behind the cut.
1. "Where do you get your ideas?"
The toilet. While the cop was writing my speeding ticket. Picking my nose.
I mean - um - I'd just witnessed an aurora borealis...
2. "You can write anytime!"
No, actually, we can't, really. There's work involved. It may look like we're just staring at our computers (and okay, often we do end up doing that too), but just like any task, sometimes when it's time to do it, you have to do it before it's too late. When we do set time aside, it needs to be uninterrupted time.
3. "Yeah, but don't you want a job that actually pays?"
Nope, I'm going to live the life of a starving artist straight out of a 90's Broadway Musical.
Seriously though, most writers know we actually have to have other aspirations. Calling our dreams impractical is very irritating, and it's also discouraging. Maybe we will be the next Stephen King. Maybe we'll publish one novel that no one picks up on now, but gains popularity 50 years down the road. Maybe we'll only ever write fanfiction. We don't know - but neither do you.
Speaking of fanfiction...
4. "You write fanfic? I don't. I like to write my own stuff."
There's definitely a difference between writing fanfiction and writing original works. But that doesn't make fanfiction bad, as long as people know where the line is drawn. (E.L. James does not. And yet for all her ripping off of Twilight, she's done quite well, so there you go. No, wait, come back - don't go that direction.) The truth of the matter is, you've probably written fanfic at some point. Ever had teachers in school tell you to write chapter three from another character's perspective? Fanfiction. Come up with a sequel to a movie? Headcanon. It can't be helped.
Writing fanfiction is also good batting practice. It helps you get a sense of what you enjoy writing, and what you don't. Of course, it may turn out that your fic writing and original writing are vastly different - that's okay, too. It's also fine if you only write fanfic.
5. "Write what you know!"
On one hand, you do have to do your homework, and many writers don't, which becomes evident. You read about a book that takes place in your corner of civilization, and it becomes quite clear that the writer spent five minutes worth of "research" on it and relied on Hollywood for the rest. There's also something to be said for writing from a point of familiarity, even vaguely.
Sadly, people seem to think "write what you know" means that if you're a grocery store cashier, you should stick to writing about other grocery store cashiers. Golfers should stick to writing about golf. The problem is that as much as a writer expands a reader's world, a lot of the time they end up expanding their own, too. And how can you expand your world if you're not allowed to leave it?
That's not to ignore the fact that many books are at least loosely based on personal situations. There's nothing wrong with that either. But it's an option, not a must.
Speaking of writing what you know, don't give writers advice when you know nothing about writing.