Inspired by Stephen King's novel "Under the Dome", Burns threatens to cover Springfield with a dome; Lenny informs him that it has already been done before. This is in reference to the plot of The Simpsons Movie (2007), when Homer's shenanigans cause the Environmental Protection Agency to cover Springfield with a dome.
When Mr Burns remembers who he is, he has an evil plot to put Springfield under a dome. This was pointed out to have been done before, which was in the Simpsons Movie, although in the movie it was the government who did the deed.