In The Simpsons episode "Cape Feare," an Itchy and Scratchy interlude turns deadly with a suicide joke that crossed the line.
In the Season 4 Simpsons episode, "The Front," Bart and Lisa read a letter that is implied to have a swear word.
The less said about the plot of the long-fabled, finally-arrived Simpsons Movie, the better; I know this instinctively, as a member of that particular segment of geekdom most psyched and apprehensive ...