Ouch (ch 68):

Minerva was going over the Transfiguration parchment due Monday, and had just marked down to negative two hundred points a fifth-year parchment with an error that could have potentially killed someone. During her first year as a professor she'd been indignant at the folly of older students, now she was just resigned. Some people not only never learned, they never noticed that they were hopeless, they stayed bright and eager and kept on trying. Sometimes they believed you when you told them, before they left Hogwarts, that they must never try anything unusual, give up free Transfiguration and use the art only through established Charms; and sometimes... they didn't.

"It's better if you don't even try" - I wonder what inspired that sentiment.