I just noticed that in a query letter I sent to a few agents I had “crises” where I should have had “crisis.” Ouch. Now I look like an idiot who doesn’t know the difference between the singular and the plural. Is it a big deal? Yes. While agents read so much slush that they might skim past such a mistake without noticing, they read so much slush that they are looking for any excuse to reject, so if they notice it…
In my day job I’ve been on a hiring committee for a position that had 170 applicants. I was looking for reasons to weed, and some of my colleagues on the committee were absolutely brutal: Rejecting people for comma-that-should-be-a-semicolon stuff. When you’re on the crowded side of the market, you can’t give them any reason to reject.
I am, however, patting myself on the back for one thing: I only sent that version of the query to three agents. This is exactly why I didn’t send a single version of the query to more. There’s always the possibility of an error that you’ll catch after sending something out.