"You and I" when it should be "you and me". Also see "He & I" in place of "him & me", etc.
Why does this actually matter? I mean, really? This is a perfect example of an obsession with 'correctness' over communication. There is no confusion of meaning here.
Well, it depends what you mean by actually matter, I suppose. It may not actually matter to say something like, "Bad grammar makes you and I cringe", but would you say "Bad grammar makes I cringe"?
While I'm in pedant mode, I remember a nice illustration of why punctuation really matters (sometimes):
Don't stop.
or
Don't, stop.
All the same to you?
You're not being a pedant, you're just confused! In fact you are confounding two completely different issues and trying to make them out to be the same.
If you read what I wrote, you will notice that I specifically mentioned that there was no confusion in meaning in the situation that annoyed Bledlow. And indeed there is not confusion of meaning to say 'bad grammar makes I cringe', in fact that is a not uncommon west country dialect usage (as Jaded noted). Just try and tell me that everyone who says that in the west country is 'wrong'...
You second example however is of a completely different situation - where punctuation can cause meaning to change leading to confusion. In that situation, as in the first, I am in favour only of avoiding such confusion.
Let's get this straight, most of what is seen as 'correct' English is simply an imposed and artificial set of rather arbitrary rules imposed by a very small, self-selecting group of C18th grammarians who were trying to 'fix' the language in place and get rid of class and regional variation. They utterly misunderstood the way languages evolve and change.
Language does what language does. You can't make it stop or behave how you want. You can only speak and write how you wish and hope, if that is indeed your hope, that others might chose to do the same.
IMHO, y'all all jus chill now, d'ya hear?