Here's a great list of mistakes that could land your queries or submissions in File 13. Some good advice here:
Some agents and publishers are fuss-budgets, so it never hurts to make your query slick and professional.
One point of contention not listed here is getting the agents/publishers name right when addressing them in a query or submission. Personally, I get annoyed when people address me as "Faun" rather than spelling my name correctly. Especially when I've responded to your query or submission personally and you have my signature right there for reference. I won't can a submission or query over it, but it's only four letters--it shouldn't be difficult.
It may seem tedious to have to write and rewrite a query letter, but it's time worth investing. The whole point is to make the agent/publisher want to read your manuscript. If they're distracted by misspelled names, irrelevant information, bad grammar, etc., they're not going to want to invest the time it would take to read your work. The same way too much backstory, bad grammar, inconsistent characterizations, etc., will make readers put down your book unfinished.