Beta readers, of course, read what the Alpha dog (the writer) has written with a critical eye to catch any major kinks in the story - grammar, characterization, plot, or otherwise. Since I cycle through my manuscript from the beginning as I write (I reread and edit and then take a flying leap forward by writing the next scene, then repeat the process) I often catch my own missteps.
But I don't see them all because, as I make changes in one part of the story, I get so engrossed in the process I don't notice how that change affects other scenes. The more I immerse myself in my writing, the more opportunity for minor flubs that fresh eyes will likely catch right away.
Here's one I found recently from my forthcoming novella, Worm (and if I hadn't I'm certain my beta readers would have):
Robyn stifled a cry and Worm swung around. Jake, waving a bottle of Jim Bean, was standing in the hall. He'd emerged from Worm's bedroom in a stupor. The man's dirty blond hair scraped his bare shoulders and a pair of boxers sagged around his waist. He blinked a few times, adjusting to the hallway's 100 watts. Jake put the bottle to his lips, took a swig, then wiped his forearm across his three day stubble.
Jake stepped back a few paces, reached behind him and pulled out a gun that had been wedged between his belt and back. The pistol waved wildly in syncopation with Jake's unsteady rocking. "I'm tellin' you. You make a move against me boy, and I'll stick this down your..."
Wha...? The guy pulled a gun from his belted underwear? Oops, no. My first draft had Jake simply showing up drunk and all hell breaks loose, he pulls a gun, etc., etc. On my second edit cycle I knew I needed Jake in the house, so he comes out of the bedroom in his boxers. But I'd forgotten that the first draft Jake was clothed. Here's the revision:
Jake stepped back a few paces and, temporarily disappearing into the darkened bedroom, reemerged with a gun. The pistol waved wildly in syncopation with Jake's unsteady rocking.
Okay, maybe not as exciting as whipping the gun out from behind his back, but a bit more realistic. But one of my beta readers suggested that if Jake was drunk, he might still have his pants on. That seemed right, so my final (?) version is:
Jake stepped back a few paces, reached behind him and pulled out a gun that had been wedged between his pants and back. The pistol waved in wild syncopation with Jake’s unsteady rocking.
That's just one example. I have others - and so do you. That's why we need beta readers. Anyone want to testify?