You know how Fox has a weird way of counting Simpsons episodes? They refuse to count a couple of them, making the amount of episodes inconsistent. The reason for this is a lost episode from season 1.
Finding details about this missing episode is difficult, no one who was working on the show at the time likes to talk about it. From what has been pieced together, the lost episode was written entirely by Matt Groening. During production of the first season, Matt started to act strangely. He was very quiet, seemed nervous and morbid. Mentioning this to anyone who was present results in them getting very angry, and forbidding you to ever mention it to Matt. I first heard of it at an event where David Silverman was speaking. Someone in the crowd asked about the episode, and Silverman simply left the stage, ending the presentation hours early. The episode’s production number was 7G06, the title was Dead Bart. The episode labeled 7G06, Moaning Lisa, was made later and given Dead Bart’s production code to hide the latter’s existence.