Plagiarism is serious business.
I find plagiarism roughly once a quarter. If I suspect it, I ask the student to bring me the sources of information that they used. If the student denies plagiarizing and I find no evidence after looking for it, there is no penalty. If I follow up and find evidence, the case is referred to a University Judicial procedure which can lead to expulsion from school. Sometimes students claim that plagiarism was unintended.