Whenever I call a submission a "sub," I get a little hungry. Which is kinda funny, since hunger has something to do with the simultaneous sub, grasshopper.
After all, the hungry writer wants to make money. Rather than send his or her submissions out one at a time, the writer hungry for success may be tempted to send those submissions out willy nilly, hoping one will land in a cash-paying market all the sooner.
Of course, many editors are not fans of the simultaneous sub. Some editors go to a lot of trouble to give each sub a fair reading. And after investing that kind of time, an editor is not happy to get a note from a writer saying , "Oops! I've already sold that submission elsewhere!"
If a writer's guidelines say "No Simultaneous Submissions," I generally follow that little rule. Mostly, because I'm afraid that a little thing called karma will catch up to me and smack me around if I don't. But also because I don't want to have to say to an editor, "Oops! I sold that piece somewhere else."
Just like I had to do the other day. In my defense, I'd sent that story out months and months (and months) ago to an anthology. I'd completely given up on it since I'd assumed that the anthology editor had given up on it, too. But I'm a hungry writer. So I sent the same story back out into the world to a magazine market that just happened to be a "No Simultaneous Subs" market.
I sent an "Oops!" email to the magazine editor and he was quite nice, actually. I may even work up the nerve to submit to the magazine again. I mean, I'm still a hungry writer.
But you can't always depend on such kindness from a stranger. Or in your case, grasshopper, an editor. So follow that "No Simultaneous Submission" rule and good karma will come your way. Or maybe just a good sub with plenty of pickles and onions and mayo.
Mmmmm. I'm hungry just thinking about it.