The Judas transfer, the act of moving from one team to their arch rivals.
No one is more hated in football than a traitor.
So, whether it was out of spite, for the money or an act of glory-hunting, here are the biggest Judas transfers in football history.
11. Ashley Cole
Dubbed "Cashley Cole" by Arsenal fans and with good reason, Ashley Cole broke Arsenal hearts when he moved across London to join Chelsea in the summer of 2006.
But the real anger came with Cole's excuse for leaving Highbury. It was reported that Arsenal's £55,000 a week contract offer left Cole "trembling with anger" and soon after he had agreed to a £90,000 per week deal with Chelsea.
10. Paul Ince
Liverpool and Manchester United do not swap players, it is that simple. Although he briefly played for Inter Milan before moving to Anfield, Manchester United fans were angry and upset when one of their former star-men donned the red shirt of Liverpool.
9. Alan Smith
Alan Smith was once a crowd-favourite amongst the Leeds United faithful and cheered his name when he stated that he would "never play for Manchester United".
But he became a hate figure when he moved across the Pennines to join United a mere months after Leeds were relegated.
8. Robin van Persie
Robin van Persie was a beloved figure at the Emirates Stadium; captain, top scorer, talisman, you name it. But that apparently wasn't enough for the Dutchman.
He moved to Arsenal's arch rivals Manchester United in the summer of 2012, winning the Premier League that very season. He has since become hated by Arsenal fans, who often heckle their former hero whenever the two play.
7. Johan Cruyff
Despite being one of the best players in the history of the game, Johan Cruyff isn't loved by everyone, especially if you're an Ajax fan.
After returning to the club that made him known to the world (he spent three seasons with Ajax), they refused to offer Cruyff a new contract. Cruyff responded angrily by signing for Feyenoord.
He eventually went on to manage Ajax after leaving Feyenoord.
6. Andrea Pirlo
Playing for three of Italy's biggest clubs isn't how most players would see their career going.
But Italian living legend Andrea Pirlo decided to play on either side of one of the most vicious rivalries in world football. He now plays his football with Juventus, a competitive rival of both Milan teams.
5. Luis Figo
One of several high-profile players to play for both teams in El Classico, Luis Figo will go down as one of the game's greatest ever wingers, unless you're a Barcelona fan.
Figo shattered the world transfer record in 2000 when Los Blancos came knocking with a $60m offer, causing many of Figo's admirers to become haters overnight.
4. Michael Owen
A Liverpool legend who once gave the fans so much to cheer about and a product of the youth team, Michael Owen had it all at Anfield.
But after leaving for Real Madrid in 2003, Owen realised that the grass was not greener on the other side, as injury after injury ground his career to a halt.
Unfortunately, Owen still had time in his career to anger every Liverpool fan on the planet when he signed for Liverpool's nemesis, Manchester United, on a free transfer, helping them to win the Premier League.
3. Sol Campbell
Spurs fans can barely even say Sol Campbell's name aloud without spitting.
The English defender burnt every bridge in North London when he left Tottenham on a free transfer to join bitter rivals Arsenal. He then poured salt onto the wounds of Spurs fans when Arsenal went on to win the Premier League at White Hart Lane.
2. Carlos Tevez
One of the many players to flock to the newly-wealthy Manchester City after their take-over, Tevez became a target for the abuse of United fans in the subsequent derbies.
The striker was a big part of the Manchester City squad which pinched the Premier League title from under United's noses, adding to the cesspool of hatred going the Argentine's way.
If you thought that playing on either side of one rivalry was bad, just remember that Ronaldo played on either side of two rivalries.
Not just any rivalries too, but two of the biggest and most competitive in the world. The El Classico and the Milan Derby, making him a very polarising figure in world football.
Luckily, neutral fans still see him as one of the most iconic strikers of his generation.