One of the production team on ‘The Simpsons’ has noticed a long-standing continuity error so glaring that it’s incredible that the show’s legions of eagle-eyed fans have never spotted it before.

Executive producer Matt Selman took to Twitter earlier this week to show fans a still image from the season six episode ‘And Maggie Makes Three’, from a flashback scene in which Marge tells Homer she is pregnant with their third baby, Maggie. In the background to the left on the wall, however, appears to be a picture of Maggie, who hasn’t been born yet. It’s taken more than 23 years for someone to point it out – the episode was first shown in January 1995.

Selman’s colleague Al Jean responded to the initial tweet, writing that the “canon is in ruins” and adding: “I hope you're happy because we just fired SEVENTEEN people for that blunder. Fired. Through. Done.”

Writer Bill Oakley also joined in the fun, explaining in a tongue-in-cheek manner that the error “will be explained in the 5th season of ‘Disenchantment’ and may involve Fry and Leela but I cannot say anymore” – referencing Matt Groening’s brand new Netflix series and their other, long-standing animation ‘Futurama’.

More: Matt Groening reveals 21st Century Fox execs asked ‘The Simpsons’ to tone down Fox News jokes

However, while it certainly does look like an oversight, some fans have explained it away by arguing that the photo in the background in the still image is actually of Lisa when she was a baby – she has the same hairstyle as Maggie – and that Maggie wears her old hand-me-downs.

The Simpsons

Others have dismissed it as just as a secret internal nod to the whole reason behind the episode. ‘And Maggie Makes Three’ starts when the Simpson family is looking through old photo albums and Bart and Lisa ask why there are no pictures of their sister. At the end, the viewer discovers that Homer keeps them at work in order to keep his spirits up while he works his dead-end job at Mr. Burns’ power plant.

‘The Simpsons’ is set to commence its 30th season on September 30th this year.

More: ‘The Simpsons’ attack on Apu criticisms slammed by viewers