Yes the Boston Marathon bombings. The police found the backpack(s) and reddit went on a crusade looking at footage and photos from before the explosion...identifying at least one person who had a similar backpack and declaring them the bomber. Then, of course, came trying to ID them. I'm pretty sure they figured out who one of them was but - shocker - it wasn't him. Neither of the actual bombers were even on reddit's suspect list.
As much as I like to make fun of it, at the time I was really hyped on the idea of a thousand strangers coming together to help. It would have been a really cool ending to the store if reddit actually helped but the whole thing was soured by people taking it too far by putting names to their "suspects"