Furnunculus

Furnunculus is the textbook embarrassment-jinx — the spell that turns the target's face into a moonscape of large, painful boils for a few hours of misery. The incantation derives from the Latin furunculus, "a small thief" — medical-Latin slang that, by some quirk of medieval medicine, came to refer to a boil. The cast does exactly what the name suggests, with the speed and ugliness scaling with caster intent.

Harry's most famous use of the jinx comes in Goblet of Fire, in the corridor confrontation that ends with Goyle's face covered in furious red boils and Hermione's teeth growing past her chin (the latter from Malfoy's Densaugeo, cast in the same exchange). The two spells colliding in mid-air and hitting the wrong targets is a small comic disaster, but Snape's response to the resulting injuries — "I see no difference," referring to Hermione's teeth — turns the scene from prank into something colder and lasts in the memory longer.

The jinx itself is simple enough: a bright red bolt, a painful prickling on the target's skin, and within seconds, raised welts and large boils blooming across whatever surface the cast struck. Madam Pomfrey can clear the boils with a standard Hospital Wing potion in half an hour, which is why most Furnunculus casts are treated as nuisance-level rather than seriously dangerous. The counter, like with most spells in this category, is the standard Finite Incantatem if applied immediately, or the relevant healing potion otherwise.

Furnunculus is one of the older jinxes in the curriculum and shows up across many corridor brawls in the books. It is not Dark Magic, but it is unpleasant enough that adult witches and wizards generally consider casting it on someone without provocation a more serious offense than the curriculum's classification suggests.