In recent times, "plagiarism detection software" has become popular in universities and colleges, in an attempt to stem the tide of plagiarised student coursework. Such software attempts to detect any copied material and identify its source. The most popular such software is Turnitin, a commercial system used by thousands of institutions in more than 100 countries. Here, we show how to fix a loophole in Turnitin's current plagiarism detection process. We demonstrate that, in its current incarnation, one can easily create a document that passes the plagiarism check regardless of how much copied material it contains; we then show how to improve the system to avoid such attacks. (Contains 7 figure and 11 notes.)