A truly original sentence might be rarer than you think. Even though we make up most of the sentences that come out of our mouths right on the spot, a ton of them have been spoken a hundred, maybe a million times before. If I say "I always wear hoodies" (which I do), even though it occurred to me spontaneously, it's totally possible that a hundred people across the English speaking world have spoken that phrase today. In fact, a Google search for "I always wear hoodies," in quotes, yields over 300,000 results. Heck, even "I always wear underpants on my head," in quotes, brings back 7 results.
I'm surprised it's that high. I mean, I've only known one person in my entire life who regularly wore underpants on his head. True story.
So every now and then, Jeremy or I will say something, then conjecture as to how many times that something has been said before. "The cat was begging me to take a bath this morning," is one I suspect hasn't been spoken too many times before, and yet my cat does, in fact, beg me to take a bath in the morning. He likes people soup, you see. He loves to drink out of the tub while we're in it. But I digress.
Anyway, Jeremy ran across a sentence while playing World of Warcraft today that he thinks may be an example of the elusive perfectly original sentence - another player had occasion to say "Seriously, fuck these eels." (He has apparently failed to take into account Rule 34).