"Your" and "you're" are actually the same problem as with "it's" and "its" with the mix-up over possessives versus contractions. The problem is that with these near-sound alikes, the kids were writing the first word that popped into their heads rather than thinking what the words themselves actually mean. I suspect adults do the same thing when they're writing and in a hurry.
This is very true. It chagrins me no end to see I've made similar errors after having posted an entry. Many times, I've modified posts to correct typos, missed letters and errant contractions/possessives.
And I know I've missed a few, too!
The grammatical errors that drive me nuts involve number agreement.
A common misuse: "A person would be....if 'they' tried to..." A "person" is singular...and would take a "he" or a "she" (or, on random occasion, an "it").
In Britain, corporate entities are plural in nature. "Metro-Goldwyn-Mayer....they....have....are...."
But in the US, corporate entites are singular: "AT&T...it....has....is...." We refer to such things as "collective nouns" requiring singular modifiers.
Misuse is absolutely RAMPANT in the media.