The Sponge-Knees Curse was a curse which presumably made the target's legs spongy, making it difficult for them to walk. It was used during the September 1999 riot that took place at a Puddlemere vs. Holyhead Harpies match. The Puddlemere supporters used this curse against the Holyhead Harpies supporters in retaliation to the Jelly-Brain Jinx.
Behind the scenes
This spell may be achieved by Vanishing crural bones and hence may be a relative of Deboning.