omigod, this has made me panic so much. I'm terrified that I could have written something down, and it's just something I remembered reading that worked its way into my brain and then fell out on a page somewhere.
It's pretty unlikely that entire verbatim paragraphs of other people's writing could have fallen out of your brain, though. And if a particular bit of your writing makes you nervous, you could always Google it to reassure yourself that the only person who said it before you was you. (This is the extent of the heavy-duty research that bloggers did on Domenech's writing: cut and pasted 2-3 sentence passages of his essays that seemed unusually mature/eloquent/whatnot for an early-20s writer with no formal journalism or creative writing training into Google to see what happened).
He's being extra weasely about it. Posted a big poor-me post on his home blog explaining that it was all his nasty college-paper editors who sneaked passages from professional movie reviews into his colums after he'd already turned them in, and he had no idea because, y'see, he never actually bothered to read his college paper. But now he's shocked, shocked!
I seriously, deeply doubt that you've ever borrowed so much as an "and" or a "the" from anyone, ever, without full attribution, and you've already shown approximately 10,000 times the professional ethics this guy has.