Although Ephorus had given a student's paper a clean bill of health, the professor had not been satisfied and she sat down to google. She found over 30 % of the paper was plagiarized from online sources!
They wrote to Ephorus to ask how this could be. The answer is rather shocking: the texts aren't identical, you see. The punctuation was changed, and the student paper often had two blanks where the source only had one. Ephorus wrote:
Um, guys? If your system can be tricked by inserting a blank after every second or third word, we might just as well flip a coin to determine if a paper is plagiarized. This does, however, confirm that the false negatives are a big problem with Ephorus. In our study with former German defense minister Karl-Theodor zu Guttenberg's doctoral thesis, which was determined by the GuttenPlag Wiki to have 63 % of the lines on 94 % of the pages to be plagiarized, Ephorus reported only 5 % plagiarism:The erroneous punctuation has implications for the effectiveness of the plagiarism scan. we [sic] will examine how large the effects are and what we can do about it.