When I first moved to New York, I worked at a legal publisher and learned the fine art of copy editing and proofreading. You'd think being a writer and English major would already be good preparation for that, and it is, but only to a certain extent. Copy editing and proofreading require an entirely different mindset than writing does. Proofreading, especially, means training your eye to focus on spelling and punctuation, on the way sentences look, rather than on extracting the information in them. Copy editing less so; you're still reading for sense, but only in a very superficial way. You're more worried about whether the sentence is clearly expressed and tightly constructed than about the overarching argument. Clearly, that depends on the level of copy editing, too, which can vary from slightly above proofreading to a substantive rewrite.
The trouble is, once you train your eye to see these things, you see them everywhere. It's a curse. The misplaced, overused apostrophe; the misspellings, the horrible grammar become a form of constant torture. Chinese take-out menus seem to be the worst, which is understandable. At least it's a translation from another language, and God knows English is an extremely confusing language with contradictory and absurd rules. But some of the worst offenders are (ready for it?) advertising copy writers. Advertising seems to have its own rules of grammar, and it's nigh unto impossible to save copy writers from their own cleverness sometimes. And punctuation? Merely decorative.
When I was still working for the legal publishers, there was a cartoon in The New Yorker that I cannot now find (and why is that? Why is it that the things I think are funny or the paintings I like never seem to be popular enough to reproduce? What do you mean, weird? What are you saying, here? Huh?) that showed a gang of people with red pens marauding across the city correcting the spelling, grammar, and punctuation of various public documents. The caption was "Rogue Copy Editors."
Here's the next best thing, thanks to Sylvia: