It is on the episode when Barney is trying not to drink anymore, so he doesn't go to Moe's anymore. So Moe is very worried about loosing Barnie, so he tries to get Homer into being like Barney. So he asks Homer to sit in the chair next to him, which was Barney's chair. When Homer is about to do it, he says that he is not gonna do it. Moe tells him that he's just like Barney, but Homer says, "No," and he burps like him. So Moe says, "See," and asks him to dance around. Look down at where Homer was sitting, and you can see that Barney's chair isn't there anymore.