In the third episode of the Simpsons in 1987, Smithers made his first appearance.

When viewers first laid eyes on Montgomery Burns' assistant, he was black.

But when he appeared on screen again he was yellow, because of what the show's creator Matt Groening says was a mistake.

The 60-year-old claimed that the character was always meant to be yellow, but animators painted him the wrong color and, because they were low on funds, they could not afford a retake.

The writer also revealed that all the characters are yellow, because he wanted them to look different to everything else on TV, and have four fingers because it is easier to draw.

Talking to TMZ, he said: 'He was always yellow, and they painted him wrong once.

'At the time we didn't have enough to do retakes, so when there were glitches and mistakes it stayed that way.‘

'He was never 'black', it was an accident.'

The interviewer then asks why all of the characters are yellow, to which Groening sarcastically replies:  'I've never noticed'. He then adds: 'It is so they wouldn't look like anything else on TV... except for Spongebob.'

And he said that the reason all of the characters have four fingers is because it is 'easier to draw'.

'That is a lot of pencil mileage if you add another finger' he said.

The 26th season of The Simpsons begins on September 28, where Rabbi Hyman Krustofski, Krusty the Clown's father, was killed off.

The show originally ran as shorts on The Tracey Ullman Show in 1987.

After three seasons, the shorts were developed into a half-hour prime time show that debuted on December 17, 1989 as a Christmas special.

The Simpsons have won 28 Primetime Emmy Awards, 30 Annie Awards and a Peabody Award.

Homer's catchphrase 'D'oh!' has entered the American lexicon while the show has become the longest-running US sitcom, the longest running American animated program and the longest-running US scripted primetime TV series.

The show's other notable catchphrases include Marge's questionable 'Hmmmmmm', neighbor Ned Flander's greeting 'Hi-Diddily Ho!' and schoolyard tyrant Nelson's 'Haw Haw!'

The Simpsons Movie was released in July 2007 and the feature-length film grossed more than $527 million.