One of the most popular and longest-running gags throughout the past three decades on The Simpsons is Bart Simpson prank calling Moe in his tavern. Set in the days before cell phones and caller ID, Bart calls the bartender anonymously and asks to speak to someone by a made-up name like "I.P. Freely" and "Seymour Butts." Moe yells around asking if that person is around and the whole bar laughs upon hearing the funny name. Moe throws a fit at the caller and hangs up as Bart laughs hysterically.
Even with prank calling becoming more difficult in recent years, this remains a classic skit in Simpsons lore because it perfectly juxtaposes Bart's mischievous behavior with Moe's grouchy personality and down-on-his-luck life. But when you really think about it, it's surprising that Moe couldn't figure out that the prank caller is the son of one of his most frequent customers. One Simpsons fan theory on Reddit suggests that Moe may have figured out it was Bart calling him sooner, but he just chooses not to do anything about it.
The theory notes that while Moe may be a lonely curmudgeon who can be hostile towards adults, but he's shown a soft spot for kids by working at a Children's Hospital. When he gets prank called by adults like Fat Tony asking for "Yuri Nator" and Mr. Burns asking for "Waylong Smithers" (not a prank name but Moe certainly thinks it is), he responds more violently and threatens them. When getting calls from Bart he's much less angry -- with the exception of the "Moe Ron" prank call because he was in the middle of attempted suicide. The best explanation as to why he's not as angry when getting calls from Bart is that he knows it's just a kid on the line.
A scene from Season 3 best suggests that Moe knows Bart prank calls him and goes along with it. After calling the tavern with the "Bea O'Problem" joke, Marge immediately asks Bart to pick up Homer from Moe's. As Bart enters the bar and asks around for Homer, Moe says intensely, "Wait a minute! I know that voice..." A nervous Bart sees him approach but Moe changes his tone and says "If it isn't little Bart Simpson! I haven't seen you in years!" Moe even asks him if he makes any trouble behind his Dad's back and Bart actually admits that he makes some prank phone calls, but not specifying to whom. Watching that scene back with this theory in mind provides plenty of evidence that Moe knows that Bart is the one calling him, but plays along by pretending he hasn't seen him in years.
Another important scene to validate this idea is when Principal Skinner makes Bart call his father. Bart calls Moe's knowing Homer is there, but while Skinner is away he asks for "Homer Sexual." Moe starts going off on the caller before Skinner gets on the line and replies: "You'll do what, young man?" Moe is flabbergasted and recognizes Skinner's voice is on the line -- a different one from the voice from earlier. Principal Skinner then asks for him to hand the phone to Homer, making it easy for Moe to assume that Bart was the one who called.
Arguably the best evidence to suggest that Moe knew all along, however, is a scene where Bart frames Jimbo Jones as the prankster and uses the Simpsons' address. Homer is not only one of Moe's favorite customers but he's also one of his best friends. Moe had to have visited the Simpsons home before confronting Jimbo, so it wouldn't make sense if he thought it was the Jones' household.
Those few scenes from The Simpsons strongly suggest that maybe Moe isn't as angry and stupid as perceived. Instead, he plays along with his favorite customer's son to make some laughs. After all, Bart's pranks aren't supposed to be malevolent in nature, it's all in good old fun.