As the others have outlined, Boston marathon was bombed, Reddit tried to work out who did it and failed spectacularly.

Reddit ended up going with a kid that had been missing for a while at the time of the run, kid was later found dead for unrelated reasons however the "investigation" by Reddit put an already extremely anxious family through even worse because instead of their kid just being missing the news/social media was now claiming their kid was a terrorist who had bombed a marathon.