We're the same! Errors in published books (even self-published ones) drive me batty. And traditionally-published? Utterly unacceptable!

While I try not to twitch over it, I know I make errors in my own posts, too. But like you said, it's more casual -- no one is paying me for this, no one is buying my posts. I do my best, but I'm human, and sometimes I don't proofread as closely as I should.