Many is the time I've heard young (or new) writers worry about the submission process.
"They'll steal my idea!"
You'd be surprised how many writers (or, actually, wannabe writers) worry about this. There are a lot of things that young writers should worry about, but this is very low on the list of such things. Grammar, punctuation, plot, ability. These are things that they should worry about.
That said, in the many years that I did submit my work, I suspected that one of my plots was stolen (at a comics company), and one was all but confirmed as stolen (at a magazine). I'll just talk about the one for which I have the most evidence. (And, no, I won't name any names. It's water under the bridge, even as pissed as I still am about it.)
Back in the 1980s and early 90s I was writing like mad. I would rarely go a week without writing a new short story, and I kept the Postal Service busy in sending out my manuscripts to magazines and anthologies far and wide. (This is one thing that I do really like about the digital age--online submissions.)
There was a particular market that I was often trying (without success) to crack. It was a horror magazine and I really wanted to get my name on their contents list. I would send stories there with some regularity. I'd heard that they had a new first reader/assistant editor who was also a writer, so I sent him one of my stories. It had a neat plot that boiled down to this:
What if everyone in the country who owned a gun was suddenly overcome with the urge to use it to shoot someone, anyone?
I've always had difficulty with titles for my stories and struggle as I might, the best I could come up with for this one was "One of These Days". I sent it out to the mag, specifically to the new assistant editor.
After a while, the story was returned, with a note from the assistant editor (I still have it). It read, "This one is pretty good, but it lacks a certain impetus, so I'm passing."
Fair enough. Unlike many other writers I've always appreciated the time an editor takes to read my work even if they don't accept it for publication. I've always had a thick skin that way. You really need that to be a writer.
Well, imagine my surprise when, a couple of months later, I get a copy of the magazine. In it, there's a brand-new story by that very same assistant editor. That story's basic premise? If you guessed 'what if everyone in the country who owned a gun was suddenly overcome with the urge to use that gun shoot someone, anyone?', then you are correct! To add a very special insult to the injury was a note on the editorial page by the publisher explaining how the author/assistant had come to the publisher with this great new story that was so good that the publisher delayed the deadline to include it in the issue. I'm positive it was written about a day or so after he put my own story back in the mail to me.
Shit does happen.