Dumbledore's Army parchment jinx

This jinx was used by Hermione Granger in 1995 as a safeguard against traitors within Dumbledore's Army. Hermione placed it on a piece of parchment signed by all D.A. members so that if any of them betrayed the group, boils would form on his or her face across the cheeks and nose, forming the word "SNEAK."

Known Uses
Although Hermione placed the jinx onto the parchment, she neglected to tell anyone about it at the time of the signing; she simply told them that if they sign, they are agreeing not to tell. When Ron showed her the newly placed Educational Decree Number Twenty-Four and claimed that she was naive for believing that by acting honourable and trustworthy will ensure the signers to maintain secrecy, she revealed to him and Harry that she jinxed the parchment as a method to both revealing any traitors and punishing them in a humiliating way, much to Ron's glee. However, as it was an external party, Willy Widdershins, who reported this meeting, the jinx did not activate on any signers.

When Marietta Edgecombe betrayed the D.A. to Dolores Umbridge, this was precisely what happened to her; she was so humiliated that she would not show her face for almost a year, and when Umbridge claimed she could not remove the jinx, Harry felt a surge of pride in Hermione's jinxing abilities. Her best friend Cho Chang considered Hermione's jinx a "horrible trick," arguing that Hermione should have at least told everyone that she had jinxed the parchment, but Harry retorted that he thought it was a "brilliant idea." This was the row that triggered the end of their romance.

Marietta had the marks even the following year, and on the Hogwarts Express Harry smirked when he saw Marietta was wearing a very thick layer of make-up but the pimples were still visible. This, along with in the previous year where Umbridge, Cornelius Fudge and even Madam Pomfrey could not reverse it, indicates 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.
 * As not even Madam Pomfrey could reverse the effects, it may mean that the Cure for Boils and Bubotuber pus, effective for removing pimples and acne, does not affect this jinx.
 * It is possible that this jinx only activates if the targets willingly break the contract, because in the film adaptation of Harry Potter and the Order of the Phoenix, Cho Chang was forced to betray the D.A. through Veritaserum, and the jinx did not activate, meaning coercion will not trigger it. However, the jinx was not mentioned at all in the film adaptation.
 * It appears the jinx requires the victim to enact the specific act of revealing that the organisation exists to a teacher in order to activate, as Dean Thomas led Seamus Finnigan, who was not a member yet at the time, to join with no consequences. Marietta did not suffer when she told Umbridge that it would be advantageous to go to the Room of Requirement; it was only when she said a meeting was taking place that the boils appear.

Appearances

 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Half-Blood Prince