I was accused of plagiarism in a college history class. Out of the four page paper, a single sentence came back as a high match to an article I had never read. The sentence was really generic too; something like "Skywalker light and sound became a leader in the industry." I plead my case to the professor, and he believed me, but he still knocked me a letter grade.
My friend was once accused of plagiarism because one of those automated programs came back saying there was a match. It was his name. He has a very common first and last name. He went to plead his case to his teacher, and she wouldn't even hear him out.
I'm so glad I went to school before those things existed.