When introducing this unaired sketch from the last season of “SNL,” Bill Hader and his fellow writers call it “the greatest trainwreck.” They’re not wrong. It’s really impressively bad. Come for the wall that almost falls on Bieber, stay to find out who that is prancing across the stage in a giraffe costume. Hader and his co-writers swear this sketch killed in the writers’ room. You tell me how exactly that’s possible.