People who had thought that they'd written the most brilliant piece ever were upset that the beta reader they'd chosen found spelling or grammatical errors and had constructive criticism to help tighten up the story.
This is why I don't comment with concrit unless it's something so glaring I can't stop myself. I used to be a professional proof reader, and a bit of a grammar Nazi -- it's so damn hard to harness those traits into something kind and gentle when I can barely keep them under control in my own writing. I give a lot of leeway when reading if I know someone isn't an English-as-first-language writer, but when it's clear that someone just doesn't care enough to put effort into it, I just hit the back button and move away before someone gets hurt.
For me personally, my beta is my lifeline. I read everything out loud so I catch my own technical errors, and my beta listens for continuity and clarity and the joy of the story. Sometimes the advice stings, but I've learned that it's like putting antiseptic on a bad papercut: sure it hurts, but the results are worth it.