Muggles' Guide to Harry Potter/Magic/Furnunculus

Overview
Furnunculus apparently causes whoever it hits to erupt in boils.

Extended Description
This is one of the classic duelling spells. Harry used it first in Harry Potter and the Goblet of Fire; he had been goaded into duelling with Draco. The spell hit Draco's Densaugeo jinx and ricocheted, hitting Goyle.

Analysis
In Harry Potter and the Goblet of Fire we learn that combining jinxes can have unexpected effects – apparently, Harry's Furnunculus curse combines with George's Jelly legs curse to afflict Crabbe with little tentacles all over his face. We can't be certain that the boils that afflicted Goyle in the earlier episode were the intended effect; the spell may well have been modified by hitting the Densaugeo spell. However, "Furnunculus" in Latin translates directly to "boil".