Grammar... sort of.

Ok, so like, I am not a prescriptivist, blah blah blah, but I do tell people how to use apostrophes for a living, so I found this momentarily funny. 

But then I found it annoying because that ain't grammar. It's punctuation, and perhaps also spelling. If it was grammar, people who make this mistake would be genuinely mixing up

CP[2.sg BE.pres.sg shit[ADJ]] (a clause with a second person subject, the copula and an adjectival complement)

and

NP[2sg.poss shit[NOUN]] (a noun phrase consisting of a possessive second person determiner and a noun).

Methinks they are not. They just don't know how to spell the right version of two homophonous but otherwise distinct forms.

That's how to kill a joke.