This is why we have ZERO tolerance in the archive for it. It happens far, far too often. That being the case, there's been a definite uptick in that with academics as well.
But this is ALSO why we tell people to cite their sources here, and warn the stories if they do not. We don't consider the leaving out of a citation plagiarism, but a mistake that can be corrected.
Of course, you don't have the option of having your work hidden and then deleted at uni for non correction