Well, I can think of two occasions where things went mildly awkward for a while.
The first was when my (then) GF introduced me to the new Doctor Who series, this being midway through the second series when Tennant was in charge. So when watching one episode together she breaks out the entire "If I had the chance, would you mind if I have sex with Doctor Tennant?"
My instantly blurted and non-thought out response? "Only if I get first dibs on him."
Granted, adding the "Whaaaat?! You just said he was Yummy!" didn't help matters that much either, but there was much awkward silence for the remainder of that ep. Much, much silence.
Number 2 is pretty straight forward. If ever it's discovered that you're hotter than your girlfriend in her clothing, you get in trouble. Even if it was her suggestion for a Halloween costume. Really, there's no way to make that realization, or the ensuing argument/discussion, any more pleasant or any less awkward.
Trust me.