A raid backfire usually occurs when some idiot posts a raid target that is either undeserving or simply a bad target using a username (on any of the *chans or other anonymous posting sites) or on a sign-up board with a username he commonly uses across the intertubes, and after a single google search the aspiring raidfag has all he needs to ruin the person's life.
A good example would be the backfire-raid on Alex Wuori.
After a raid backfires it is common for the raidee to attempt to remove all evidence of his epic failure, usually resulting in him getting hit again for his attempt to take down ED or his ED page after discovering it. This happens on forums all the time.
The problem facing the genius masses of today is, they believe that whomever they are asking is their personal army and will usually not take part in the raid at all themselves, which generally causes the bored anons of today to attack the person calling for the raid instead of the intended victim but this is usually only done by aspiring /i/nsurgents who believe they are doing the right thing simply by attacking the person who is calling for blood.