It's for its and vice versa are pretty common mistakes by people who do (not ought to, but do) know better. Unfortunately that knowing better doesn't always seem to make it to the fingertips. The same is true for your/you're, there/their/they're and the like. This is exacerbated by the simple fact that we are all our own worst proofreaders. What I find particularly funny is when someone couches a complaint with the "we have a perfectly good word for that, we don't need another one" argument when the word being complained about has a subtle difference from the one being proffered as the correct choice and the same person turns around and complains about subtle distinctions being lost when two words are confused.