1. Don't misspell the editor's name. I've done that. Last summer, while on pain medication after major surgery, I completely changed the editor's first name. How embarrassing. The frustrating part is I discovered the mistake about 5 minutes AFTER I pressed send on the electronic submission.
2. Don't forget the SASE. I've done that too.
3. Don't put one editor's name on the cover letter with the wrong publishing company name and address. I have not done this but it has almost happened.
There are so many more goofs that make me nervous just before dropping my hard work in the mail chute or hitting the send button. Anne Shirley from Anne of Green Gables says, "I promise I’ll never do it again. That’s the one good thing about me. I never do the same wrong thing twice." I've learned to make a list of things to check and I read my cover letters out loud to my husband.
When it comes right down to it, if an editor wants to publish my work, maybe they will overlook my little goof-ups and my nerves are not going to keep me from submitting.