Any "accident" has nothing to do with the type of source or its author.
There have been cases ( and you can look this up in various universities and colleges rather than asking me to prove it) where the students actually use the "lent paper" by the professor and have it written up by someone else, using the same source material. It is a business the other academic writing companies call "paraphrasing services" and these are all based on previously written papers "borrowed" from the professors.
The professors always catch on and penalize the student who did it, but refuses to admit it. Then the student chases after the hired writer who botched the job since he was caught using the plagiariazed paper. I have never been involved in these instances as I draw the line for academic integrity at some point. I am however, in the know about some freelancers (not you!) and writing companies that do this kind of work. I rejected such work offers in the past.
Thankfully, I am not an active academic writer anymore so I am no longer faced with these decisions. I am out of the academic business totally so you should not consider me a threat to you and your mode of work at all. Which is how you seem to eventually view any other active person on this board. Scared because you cannot keep up with the modern way the business is done I guess? You must be too old to be in the business already which is why you are no longer relevant and familiar with how the students actually get around the old blocks and use new ways to cheat the system. Modern way of doing things? Not really your style is it? Hence your defiance at any explanation that proves you to be wrong.
I do still enjoy discussing the business though. Which is why I am explaining the method by which these "accidents" occur to you. I hope you don't get an aneurysm reading my posts. My disclaimer stands.