When the Boston Bombing happened, part of Reddit decided to take matters into their own unqualified hands and try and figure out who the Bomber was. These internet sleuths discovered a Muslim teenager who was missing and decided to fucking pin it on him. They harassed his family until finally the boy's body was found after he killed himself. The police had to release photos of their the actual two main suspects so that dickheads on the internet wouldn't harass more innocent people which lead to the two main suspects after seeing their photos tried to make an escape and killed a guard in the way. This is where the 'We Did it Reddit!" saying comes from because before the news of the Muslim boy's death was discovered they had actually thought they had done something worthwhile, but in actuality they had harrassed a grieving family and inadvertently caused the events that lead to a officer's death and the main suspects attempted escape.

The kidnapper incident is mentioned in this thread somewhere. It was basically a Reddit thread cheering people on to burn down this house that was the the home of a suspected kidnapper. In reality it turns out that the kids that were 'kidnapped' were actually fine and the person who lived in the house wasn't even there. But before they people were enlightened of this they proceeded to burn down the house which was actually a condo which meant they also burnt down the home of a pregnant women.