I know the difference between "your" and "you're".
I know that language is fluid and always changing, and "you're" and "your" is irrelevant.
I understand cultural differences and change in language, both written and verbal.
Hence why we aren't still speaking Latin or 17th century English, and why there is no cut-off date between the two (and others) when they 'magically switched.'
I know if someone has difficulty deciphering between an incorrectly used "your" and "you're" in a conversation, then they probably have severe comprehension problems.
I know that someone who picks out a "your" and "you're" mistake has little or no understanding of my second or third point.
I know that someone who picks out issues with grammar and punctuation probably doesn't realise there is now a difference between writing for print and writing for the digital arena (in the professional world).
I know there is also "youre" (see point 6).
Wait... that's seven things - the question did only ask for one. Oops.