This has only happened once before, and I recognized the truth much earlier, so as to be much less exposed.
Today, I all but accused a student of plagiarism who I am now convinced did not plagiarize.
It was as fishy as a koi pond, to be sure...because I am experimenting with not maintaining an attendance policy, the student had not attended a class except to take an exam since early February. He had access to decent notes from a friend, and so did well on them, but had only gotten the assignment two days before the paper was due, then chose to write on the longest text we had yet studied, for which he was entirely absent.
The paper I received wasn't, but the kicker--it had the wrong course number in the header, a course number not offered at this university. A little googling revealed a university course at another institution for which this assignment would've been entirely appropriate and I thought I had him.
I have confronted many many plagiarists in my time, and even the most hardened liars have a pretty clear tell when they've been caught. This guys was either cool as ice, or clueless--he legitimately thought that this was the course number for the class--I know, because I asked him, and he looked at me as if I were crazy and gave the same wrong answer as on his paper. I then asked him to forward me a copy of the paper, which the MS Word properties clearly indicated had been started at 7pm the night before it was due, and printed 6 1/2 hours later.
And if the student was smart enough to fix the properties of the document, he'd be smart enough to catch the course number.
The other piece of evidence I was relying upon? He labeled it "Final paper," which it wasn't. It was only paper #1. As I pulled out the assignment sheet to show him, there it was, on the top of the page, my own handout: "Final Paper Assignment."
Mud in my eye.