1. You are confused about the distinction between fiction and non-fiction.
This is an absolute deal-breaker for me. You can write one or the other. Memoir is non-fiction, as a general rule, even if you're taking poetic license on some elements.
How you will avoid this: do not use the word memoir AND novel to describe your work. Do not use the words non-fiction AND novel to describe your work. If you are unclear about what you are writing you should not be querying.
2. A non-fiction proposal that is too broad to be reasonably addressed in a book length work. "Peace in the Middle East" is an example. "Ending Violence" would be another. "Why Barbara Poelle Loves Vodka" would be on that list.
How you will avoid that: Be able to answer the question "what problem does your book address." The problem isn't war in the middle east, it's the things that lead to war. You need to address the things that lead to war.
3. Querying for a second, third, or fourth novel in a series that was published by someone other than who you want to publish with now.
How you will avoid this: moving a series mid way through is VERY hard. The only reason a publisher will be interested in picking that series up is if you've sold REALLY well. If I run Bookscan numbers on your title and come up with anything less than 10,000, it's probably a non-starter.
4. Querying for a book with what I think is a ridiculous premise
How you will avoid this: you can't and should not. The worst thing that will happen is I send you a rejection letter. Other agents can and will love your work. If you don't believe this, you should know that when my colleagues and I have an after-hours Read the Queries party there's always at least one query that half the room loves and the other half doesn't get at all.
5. Telling me in detail about why another agent didn't pick up the book.
How you will avoid this: Don't ever mention what anyone else thinks or did in a query. Even if you think it increases your chances with me. It Does Not. I'm not likely to think a book a top-notch agent couldn't sell is something I'll run the table with. Let me fall in love with your story, not your querying history.
6. A query that I literally did not understand. (All the words were in English, I double-checked)
How you will avoid this: Have someone beta-read your query, and not someone who depends on you for their livelihood or their home. In other words, objective beta readers. If they can't answer the question "who is the hero" in ten seconds, you have a problem. If they can't repeat the precipitating incident or what's at stake in the novel in 30 seconds you have another problem.
7. A query about a book that has a political agenda.
How you will avoid this: story first. Any point you want to make should come from the story being told. And please, leave your intended message OUT of the query. I reject these books without even reading the pages, cause I've NEVER actually liked any book I've read that had an overt message.
8. Word count is too short.
How you will avoid this: know the word count requirements for novels. Anything under 80K gets the fish eye from me. Anything under 50K gets an auto-reject.
9. The query talks about the theme of the book
How you will avoid this: don't do it. Tell me about the story. Tell me what matters. Tell me what happens that wasn't supposed to and how the characters are dealing with this challenge. Do NOT tell me about the theme. (See also #7)