Scoff at them

My grammar professor (who I think might be a spy, but that story another time), showed us how to identify and avoid using sentence fragments. Which I already knew how to do. As you can see. Anyway, he said that they are sometimes used by "professional writers" who "know what they are doing" and "use them on purpose". My professor said for the purpose of our class, however, they would not be acceptable. He said we should "recognize them, and scoff at them." Scoff at them, I swear he said it. I intended to follow his advice. Unintentional fragments, beware.