" ...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)
The Backfiring Jinx has unknown effects, but can certainly 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).
Commentary
Notes
Since Regurgitating Toilets were also found around the same time as the Backfiring Jinx, Rowling may be using a little bathroom humor to make a few puns: the Ministry "got wind" of the Jinx which caused some type of "backfiring," possibly implying it gave victims a bad case of digestive gas.
From the Web
"Backfiring Jinx" from Harry Potter Wiki
Pensieve (Comments)
Tags: backfire