I don't think anybody living today can say they've never accidentally plagiarized something, whether it was on purpose or by accident. We Americans live in a culture that seems to revel in stealing the work of others and either turning it into a joke or a competing product.
Lets say you've just finished reading something that really got your imagination flowing, like Steph Meyers' The Host. When I read it, I became very interested in alien invasion stories. To be clear, I don't mean I started researching alien abductions and close encounters; I simply enjoyed the idea of aliens coming to earth for some mysterious purpose. This was the basic idea behind many episodes of the original Twilight Zone in one form or another, and sometimes the aliens turned out to be us.
After reading The Host, I didn't start trying to write a better alien invasion/romance story. I don't want to copy anything, especially not something filled with romantic mush. Instead I started trying to think of new spins on the idea.
What if the aliens are actually ancient Earthlings returning to Earth to capture a relative who has no idea his ancestors flew off in a saucer?
What if aliens land and do nothing but lie about everything? Though they already did this in a Dr. Who. SPOILERS: The alien invasion is just a cover while the real aliens subvert all of the governments of the world. END SPOILERS.
If you read enough sci-fi short stories, inevitably you're going to read something about one kind of alien invasion or another. Star Trek has done it many times, Isaac Asimov has done it more. In one episode of The Next Generation, they even had the humans dress up like aliens and move around among the aliens. Why would they do this?
To make sure it's safe for the rest of them.
I was gonna have a clip from Signs up there where that army general says that line ominously, but it seems nobody has put it on YouTube. But imagine how cool it would have been if I had had it!
Anyway, the point it that my interest in The Host didn't result in me stealing any characters, events or plot hooks and writing them into my own story. My English teachers, and probably yours too, were very adamant that we do not plagiarize!
From an academic perspective, they're more concerned that you give credit where credit is due; listing sources and whatnot. As a creative writer, it might be a little harder for some people to notice that you've stolen something, but eventually somebody is gonna figure it out, and you're gonna look like an ass.
“You obviously just took sections out of the Harry Potters and one of the Artemis Fowl books, changed the names and added steampunk laser guns!” These are the last words you hear before Scholastic Books sues you. Probably.
“But William! I'm a casual writer! I'm never going to get my work published! My stories are going to live on the internet forever!”
You have to remember that plagiarizing is wrong, regardless of whether or not you get paid for it. Why is it wrong? Some people don't understand why they can't make a book mashup like they do for those Nirvana songs. Yes, you have to watch at least a minute of the following video.
To answer your question, it shows you're lazy. Why would you bother writing something that you're not going to bother to write? You're wasting your time if you do this, plus it leaves you open to pot-shots from the internet community. Maybe your mom has never read Fahrenheit 451 and so misses your clear plagiarism of seashell earbuds, but you can bet your right hand that some guy online has, and he's gonna call you on it.
I knew a girl who would take any idea that “looked cool” and Blend-Tec-blended it into her own stories. It could be anything. Books, movies, comics, television, conversations. After a while you just kind of hesitate to tell her anything because you know it's gonna wind up in her story, and she's probably gonna get paid for it.
There's another kind of plagiarism that affects many people, perhaps even more than the deliberate plagiarizers: Accidental plagiarism!
Lets say you come up with a brilliant idea. You don't know where it came from, but you know it's all you. You've decided to write it down, or draw it, and somebody walks by your desk and says, “hey that looks exactly like Jack Skellington!” Or somebody reads your story and says, “This is just Speed on an airplane instead of a bus!”
I'm not sure why humans tend to do this. Other creatures might do this too, but every time I ask the dog, she just looks at me. We'll probably never know.
Anyway, I think this usually happens because we're tired, or we were half asleep in front of the TV when Twilight Zone was on. It probably comes from bad memory recall. After three years in digital art class critiques, you realize how quickly people compare things to other things. You also realize how hard it is to be original.
So I guess my point is that you should always strive for originality. Don't settle for making something “like” something else. Be creative in your creative writing.
Also, you get bonus points if you can guess the name of my favorite photo blog.