It happens. It's expected. By the second or third editing pass and a few critique partners and beta readers' feedback there are still an amazing amount of flubs that eek by. Little bittie bastards, I call them. Minor things that spell check won't catch and my brain (and the CP's and betas' too, apparently) seem to auto-fix as we read. For example, I had an errant spotting of the mythical item called "lip floss" instead of "lip gloss" hanging out in a YA ms that had been through... well, I won't tell you how many smart people had read that thing and never caught it.
Then the agent passes through and catches a lot of those little bittie bastards, and usually the agent passes through it again to catch the little itty bittie bastards. At that point we all feel pretty secure, and the ms goes out on submission. Hopefully it gets picked up (hooray) and then the editor jumps in.
I've talked before about the editing process when you've got a real pro on your side, so I won't go into that here. Once your ms has passed through the editor it should look amazing, right? It should have a nice veneer on it like an Edwardian teak desk, right? Well... sure, it kind of does.
But there are times when that desk looks awesome but perhaps it's missing a drawer. Sometimes really, really obvious things that completely undermine a scene have slipped past you, your CP's, your beta readers, your agent and even your editor.
How can that be? Hopefully it's because all the people above were so lost in your storytelling that they simply accepted what you're saying and rolled with it. Lost in the moment, they didn't realize that the moment didn't have any right to exist in the first place.
My example - my copyeditor found a spot in NOT A DROP TO DRINK where my MC is finally letting her emotional walls down and taking a moment to study the face of a sleeping friend in the flickering shadows of firelight. Awww... it's really sweet. It's pretty much the first time she has acknowledged to herself that she actually cares about another human being. It makes people tear up, which I guess blurs their vision a little, because they forget that the scene is taking place in a concrete basement, it's past midnight, there's a blizzard outside, and they just shut the door to the cast iron stove a paragraph above so.... technically my MC can't see jack shit.
Yeah... need to fix that.