When Harry takes it, a several unusual coincidences occur, which I don’t think can be attributed to confidence alone:

• Harry decides to visit Hagrid, not knowing he would meet Slughorn on the way
• Harry accidentally ends Ron and Lavender’s relationship
• Harry accidentally ends Ginny and Dean’s relationship
• Filch forgets to lock the main doors to the castle
• Harry decides to detour through the vegetable patch, not knowing Slughorn would be there