1. misused words.
Writing gentile when you mean gentle, or maybe genial, is a huge red flag. I pretty much stop reading when I find them. Words are your tools. Misusing them is like showing me an ice sculpture you "carved" with a hammer.
How you will avoid this: proofreading. If you read your query out loud you'll find the homonyms (most likely.) If you don't, well, that's a different problem that isn't going to be solved at the query stage.
2. the number of free downloads of your story is less than 100,000
The difference between how many people will pay $25.00 to buy your book and how many will say "yes" to "it's free" is a gap the size of the Grand Canyon. I don't plan to spend any time explaining this to you.
I'm not all that keen on books that have been "market tested" anyway. I have confidence in my commercial taste. Maybe other agents feel differently.
How you will avoid this: Just tell me about the book. Save the bad news of your hamhanded marketing efforts for later.
3. "I'm following your query requirements exactly" and sending everything but what I specifically ask for.
How you will avoid this: I understand that every agent asks for different things. It's easy to get mixed up, send the wrong thing. If you just leave out the part about how you explicitly follow directions, you're better off. I've got a low tolerance for sloppiness right now. Make it easy on yourself.
4. Non-fiction book written by someone with no qualifications or education in the field.
How you will avoid this: you can write whatever you want, and publish it too. What you can't get is a publishing deal. The answer to "why should anyone pay attention to what you say" is something you need to answer with credentials, not opinions.
5. Literally NOTHING about the plot in the query letter. NOTHING.
How you will avoid this: Ask someone to read your query. Then ask her/him "what is my book about?" If they can't answer the question at all, revise. It helps to have the name of the protagonist and the choice they face, or the problem they need to overcome.
If there's no plot in the query, it's an automatic pass from me.
6. Telling me where you want your book submitted
I don't take well to authors who send me a query with a submission list. Time enough for that if I like your book. And unless you've worked in publishing, chances are you don't know enough to be making up submission lists.
How you will avoid this: keep your query about your book.
Bottom line: tell me about your book. If you can do that, you've done enough. Don't try to entice me with marketing numbers or your ideas on what make you special. Your writing should be special.