Plagiarism!

I wrote a little play involving Iago and Othello in the afterlife. At the gates of hell, actually. And I wrote it in verse. So someone asked me if I had worked in any lines by Shakespeare. I didn't think I had, but I had written it a while ago, in a hurry. So I thought I would feed the play through some online plagiarism-detection software. And it found something. But not from Shakespeare. It found this:

"It’s a lesson that I learned the hard way."

I thought to myself, that's practically a cliche, anyway. How can that be plagiarized? But it turned out they only had an 80% match. They had:

"Lessons I learned the hard way readings: Some modest advice for graduate students"

This plagiarism detective
Seems to be defective.