Chelsea named a player from another Premier League side on their team sheet for their Carabao Cup third round tie with Brighton on Wednesday night.
Mauricio Pochettino's side got back to winning ways courtesy of a solitary strike from Nicolas Jackson in the second half.
Jackson, scorer of Chelsea's first goal in September, also had a goal wrongly disallowed for offside.
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The Blues will now take on Blackburn Rovers in the fourth round after the Championship side beat Cardiff City 5-2.
But Chelsea will need to avoid a mishap they made at Stamford Bridge ahead of the clash.
Bizarrely, Brazilian Andrey Santos was named in the starting XI on the official team sheet handed out to journalists in the press box and those in hospitality.
Santos, signed from Vasco da Gama for £11 million, was listed as the No.20 despite having been loaned out to Nottingham Forest for the season.
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He has played just once so far, playing 90 minutes in the EFL cup defeat to Nottingham Forest.
As per The Sun, the error was not made on the official graphic posted by Chelsea or in the list given to officials.
But it was then made abundantly clear that the actual line up was supposed to be feature summer signing Cole Palmer, who took the No.20 shirt after his £40 million move from Manchester City on deadline day.
There was an even bigger mistake made at Stamford Bridge as the linesman made a shocking offside decision in stoppage time.
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Striker Armando Broja raced in behind from an Enzo Fernandez pass but the linesman raised his flag and a free-kick was awarded to Brighton.
This was despite the fact that Broja was in his own half and was quite clearly onside. The decision enraged fans and resulted in Mauricio Pochettino being shown a yellow card.
Featured Image Credit: Getty & Andy NaylorTopics: Chelsea, Premier League, Carabao Cup, Brighton And Hove Albion, Cole Palmer