This has been posted all over the internet. It's not really funny...it barely even makes sense (but see here for a bit of an explanation). Yet it just pops in my head all the time. I've stopped saying anything interesting or even mildly unbelievable, just in case someone says "oh, really?" to me, causing this:
Sometimes it's not even caused by anything. I'll just be sitting there, or trying to get to sleep, when suddenly:
GET THE FUCK OUT OF MY HEAD!
Well, at least there's this: