" ...we got wind of a nasty backfiring jinx down in Elephant and Castle, but luckily the Magical Law Enforcement Squad had sorted it out by the time we got there."
-- Arthur Weasley (HBP5)
Exact effect not mentioned, but can be nasty.
References from the canon
The Office for the Detection and Confiscation of Counterfeit Defensive Spells and Protective Objects got wind of one of these in Elephant and Castle in July 1996 but it was sorted out by the Magical Law Enforcement Squad before they arrived on the scene (HBP5).
Jinxes and hexes