Plagiarists have no idea how much they don’t know, and no clue about how much we know about our own subject as well as how much we know about what they don’t know. The ones that always amuse me most are the students who think they’re being clever by using a book 80 or 100 years old. Google books is now making that scheme pretty transparent, but it just kills me that 1) they think that academic interests and writing styles aren’t subject to change over time, and 2) that it’s not patently obvious when they plagiarize something written by a fusty academic writer from the 1920s or 1930s (or even earlier) and try to pass it off as work by an early twenty-first century college student.
Effective plagiarism requires more dedication and ingenuity than just completing the assignment honestly. I sincerely wish my students would take the easy way out and just write an honest paper!