I sometimes do research as I’m editing a client’s book. Sometimes this research leads to unexpected discoveries, like a little piece of inadvertent plagiarism. I have pointed out and altered bits of plagiarism in clients’ novels. I note, having read a client’s contract, this clause:
“Any discoveries of plagiarism will result in immediate termination of the contract and will require the return of any payments made. Plagiarism includes: copying text from the Internet, other books …, and work written by you for another publisher (where they hold the copyright). Be advised that (the publisher) employs several software programs designed specifically for detecting Internet plagiarism.”
(Yes, it is possible to plagiarize yourself!)
So remember that a tiny piece of explanation copied from the Internet and not attributed can get you in serious trouble.