What would happen if the Furnunculus Curse and the Jelly-Legs Curse were used on someone at the same time? Would each curse just simply retain its regular effect, or would there be a new combined effect? If the latter, what would the new effect be?
It seems to create little tentacles all over one's face.
"Thought we'd see what those three were up to," said Fred matter-of-factly, stepping onto Goyle and into the compartment. He had his wand out, and so did George, who was careful to tread on Malfoy as he followed Fred inside.
"Interesting effect," said George, looking down at Crabbe.
"Who used the Furnunculus Curse?"
"Me," said Harry
"Odd," said George lightly. "I used Jelly-Legs. Looks as though those two shouldn't be mixed. He seems to have sprouted little tentacles all over his face. Well, let's not leave them here, they don't add much to the decor.".
- Harry Potter and the Goblet of Fire, Chapter 37 (The Beginning)
So it seems like they did mix and it created the effect of spawning little tentacles all over Crabbe's face.