There are a huge number of awesome comment!fics that have come out of
maychorian's
party, but this one I have to rec, because, dude:
Insult to Injury, by
maskedfangirl.
Prompt/Summary: Dean and Sam hunt a witch, and they nearly catch her with the help of Castiel. She escapes, super-pissed and casts a powerful spell on the falling angel. The spell? Any time either of the brothers say something remotely sarcastic, or hurtful (because that's how they can be, especially Dean), Castiel gets hurt, physically. Extra hurt if Castiel doesn't understand what their talking about (aka pop culture references). They don't get it first, until one brother gets all pissy and hurtful. Then guilty bros and schmoop time?
It is everything it says it is and more. It's in eight parts because it outgrew the comment medium and is actually something like 3700w long, but it's so worth reading it's not even funny.
Seriously, though, if you like Castiel,
go poke around. There is some amazing fic over there.