How I Met Your Mother Season 5 Episode 11
Last Cigarette Ever
When future Ted is telling his kids about how Robin came back from her job upset, he accidentally let's it slip that Robin went up to the roof to spend time by herself. He finally admits to his kids that the gang were the occasionally smokers, admitting it one member at a time. It starts with Robin, who let's Marshall bum one. On his way home Marshall picks up a pack only for Lily to hop right back on the bandwagon. Marshall tries to quit, but when his old boss, Arthur (Bob Odenkirk), from "The Chain of Screaming", becomes his new boss at GNB, he finds himself bonding with his boss over cigarettes and can't quit. Ted and Barney eventually feel like the smoking is dividing the group, so hop outside to smoke with the other three. The group all decides to quit together.
Meanwhile, the only person with any actual adventure outside the smoking was Robin. Robin was frustrated by her new co-anchor, Don, who's a perpetual morning show anchor who's given up on ever making it to a network (despite being on one for a short weekend). He won't even wear pants to work and often stops reading stories in the middle of them. What's worse is this is all when Robin is trying to quit cigarettes. She tries to prove him wrong by getting the mayor on the show, but when he cancels, she starts to see that maybe he's right. He tries to get her to smoke a cigarette on air unless someone calls. That's when the entire gang, trying to quit, calls her not to smoke. She heads home to them and joins them on the roof.
Back on the roof, they all try for one last cigarette, only for Future Ted to reveal that none of them stopped smoking that day. Ted would quit two weeks into dating their mother, Barney would quit in 2017, Lily when she started trying to get pregnant, Marshall when he had his son, and Robin in 2013.