It's difficult to differentiate between "errors" and usage though
Even going back just a hundred years (maybe more to be fair, we'll call it a hundred and fifty) there is a huge difference between how people even spelled words. Were they wrong? Right?

The "long s" stands out as an example, of course.

"Could of" does bug me, but the vast majority of the time I find that people who use that end up realising that it's just plain daft, but when you think about it, it does have a certain amount of logic to it. Written language has always evolved through the written interpretation of vocal words, and I wouldn't be surprised if, with a little digging I could find a few similar examples from many years previous.
(Captain Skippy pooed in a bag and left it on your doorstep, Fri 2 May 2008, 19:20, Reply)