Dumbledore's Army parchment jinx

This jinx was created and used by Hermione Granger in 1995 as a safeguard against traitors within Dumbledore's Army. Hermione placed it on a parchment signed by all D.A. members so that if any of them betrayed the group, boils would form on their forehead in the word "SNEAK".

When Marietta Edgecombe betrayed the D.A. to Dolores Umbridge, this is precisely what happened to her. Her friend Cho Chang considered Hermione's jinx a "horrible trick", but Harry Potter retorted that he thought it was a "brilliant idea"; this was the fight that triggered the end of their romance. Marietta had the marks even the following year, indicating that a counter-jinx could not be found or that Hermione refused to give it to Marietta. The boils did fade with time, but left scars.

Behind the scenes

 * The jinx may be related to the Furnunculus hex, which causes boils to erupt on a person wherever the spell hits.