Former Manchester United manager Sir Alex Ferguson once laid into one of his players after he swapped shirts with a Manchester City player.
Ferguson was a know disciplinarian and not a big fan of shirt swapping. So when Ruud van Nistelrooy walked into the changing room with the shirt of rivals City, the Scot absolutely lost it.
The game in question was Rio Ferdinand's first derby as a United player after he signed in a £30 million move from Leeds.
The fixture was City's last derby at Maine Road before moving to the Etihad Stadium and Kevin Keegan's side claimed a historic 3-1 victory.
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Shaun Goater bagged a brace, with Nicolas Anelka also on target. Ole Gunnar Solskjaer netted United's goal but it was his strike partner who courted controversy after the game.
"We got beat 3-1 at Maine Road with Shaun Goater scoring two goals," Ferdinand recalled on a FIVE podcast episode featuring Vincent Kompany.
"That was the first derby I played and the moment I realised how big it was was in the changing room after.
"We walked in and the manager shut the door. It was quiet and then all of a sudden he just erupted.
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"Ruud van Nistelrooy, who was the God then, walked in with a City shirt in his hand and I remember Sir Alex Ferguson just absolutely unloaded on him. He destroyed him, saying: 'If I ever see any of you walking with a City shirt ever again you'll never play for this club.'
"That's how he was, he was just mad. That's when I sat there and thought: 'This is a big game, isn't it?'
"I didn't realise before that game and ever since then, whether you win more or less, I knew that week leading up to a derby was all or nothing."
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Van Nistelrooy was United's main man up top at the time but that didn't stop Ferguson from giving a dressing down.
According to former United captain Gary Neville, Ferguson stopped his players from swapping shirts in domestic games as a result.
Van Nistelrooy scored in the return fixture at Old Trafford but Goater, on as a substitute, salvaged a point for City with a late header.
In five seasons at United, Van Nistelrooy scored 150 goals in 219 appearances for the club before joining Real Madrid.
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He and Ferguson butted heads in his final days at United and in 2011, Van Nistelrooy revealed he went out of his way to apologise to the Scotsman for his behaviour.
The ex PSV and Real Madrid man sealed a return to the club this summer, joining Erik ten Hag's coaching staff.
Topics: Sir Alex Ferguson, Manchester United, Ruud Van Nistelrooy, Manchester City