Tensions often boil over on a football pitch, but it's generally seen as the done thing for players to train their sights on the opposition.
That's why you had to pinch yourself when Everton team-mates Idrissa Gueye and Michael Keane squared up to each other in the 13th minute of their side's 1-0 win at Manchester United on Monday night.
What Kieron Dyer said to Lee Bowyer in their infamous on-pitch scrap
It is not the first time that team-mates have come to blows during a Premier League fixture, though, with arguably the most famous example occurring back in April 2005.
Newcastle United had already endured an afternoon to forget as the clock ticked down on their home game against Aston Villa, with the Magpies trailing 3-0 and reduced to 10 men following Steven Taylor's red card.
Midfielders Kieron Dyer and Lee Bowyer both started the contest, but were not exactly the best of friends having often been competing with each other for a place in the Newcastle starting lineup.
With eight minutes left, as Shola Ameobi looked to launch an attack for the hosts, Bowyer accused Dyer of not passing to him.
"It's because you're s***," came the response - and all hell broke loose.
In front of more than 52,000 fans at St James' Park, Bowyer marched up to his Magpies team-mate, grappled with him and started windmilling.
I was trying to let him punch himself out, Dyer later said. But by the time the fourth one came in, I thought F*** this and launched one back.
After players from both teams stepped in to separate the pair, both players were sent off - despite Alan Shearer protesting that it was a harsh decision - to leave Newcastle down to eight men.
Dyer and Bowyer continued to clash as they went down the tunnel, before they were both hauled into the post-match press conference and made to apologise by manager Graeme Souness, who reportedly challenged both players to a fight himself.
Bowyer was hit with a seven-match ban, in part because it was his second red card of the season, while Dyer was suspended for just three games.
Remarkably, Newcastle retained both players beyond the end of the season, with Bowyer joining West Ham United the following summer and Dyer leaving in August 2007...also for the Hammers.
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