But, as Selman has spotted, there's already a framed photo of baby Maggie in the background.
"Maggie is in photo on wall behind Marge telling Homer she's pregnant with Maggie," he points out, tweeting a screengrab of the moment.
The tweet has already racked up more than 54,000 likes and 13,000 comments, causing quite the stir among vintage Simpsons lovers.
Yeardley Smith, the voice of Lisa Simpson, replied: "Oooops".
Producer Al Jean wrote: "Canon is in ruins".
He later joked that people had been sacked over the error.
"I hope you're happy because we just fired SEVENTEEN people for that blunder. Fired. Through. Done," Jean posted.
Eagle-eyed fans of the long-running animated series can't believe they missed the gaffe, which first ran way back in 1996.
Some have other theories to explain the framed photograph.
