WHEN YOU'RE (NOT YOUR) THAT GUY

I like this apart from its (not it's) obvious Americanism. It reminds me of someone ...

People who fix other people's grammar are often called pedants, nitpickers, or grammar Nazis.

It's (not its) annoying because:

• It misses the point.
• It interrupts the flow of the 'conversation'.
• It can be arrogant and bullying. 

If a message is understood then criticising a simple typo shows you care more about showing off than actually listening and the interruption can be construed as being rude.

What's funny though is that the grammar Nazi often misspells a word or messes up their own punctuation in the very same comment that they publish.