Alexis Sanchez isn't the only one.
On Sunday, news filtered through that Alexis Sanchez was edging closer towards becoming a Manchester United player. Being left out of the Arsenal squad for their clash with Bournemouth appeared to be the final nail in his Gunners career ahead of a headline-grabbing move to Old Trafford. With Manchester City walking away from the deal after being unwilling to meet Arsenal's asking price, United were able to swoop in and negotiate with the Chilean.
Sanchez becoming a United player will certainly be one of the most controversial Premier League transfers of all time, but it's just the latest in a growing list of contentious deals involving players moving to a club's rival.
Here are eight of the most memorably shocking.
Robin van Persie (Arsenal to Manchester United)
In August 2012, Alex Ferguson pulled off one of his most impressive transfer coups when he persuaded Arsene Wenger to let Robin van Persie join Manchester United. The Dutchman was insistent on joining United, leaving Wenger no choice but to let a player - fresh off scoring 30 league goals for the Gunners in the previous season - join their bitter rivals.
It proved to be a shrewd piece of business for Ferguson as van Persie scored 26 Premier League goals to clinch one final title for the Scot before his retirement.
Ashley Cole (Arsenal to Chelsea)
Another hugely controversial exit from North London, Ashley Cole joined Chelsea in the summer of 2006, with William Gallas moving in the opposite direction. As one of the best left-backs in the world at the time, it was a dramatic transfer, one that ended an absorbing saga that began when Cole met Jose Mourinho and Chelsea's chief executive Peter Kenyon at a London hotel behind Arsenal's backs.
Cole went on to win the Premier League title, four FA Cups and a Champions League during his eight-year spell at Stamford Bridge. Arsenal, of course, got Gallas in exchange, the Frenchman spending four years at the Emirates before moving to North London rivals Tottenham.
Andy Cole (Newcastle to Manchester United)
Different cole, different era, same level of controversy. Kevin Keegan famously addressed irate Newcastle fans on the steps of St James' Park after reluctantly sanctioning the sale of Andy Cole to United. At that time, in January 1995, Newcastle and United were title rivals and Magpies supporters were understandably furious that their club had significantly strengthened Ferguson's side by selling them a striker who had scored 68 goals in 85 appearances during his time on Tyneside.
After initially struggling to score regularly at Old Trafford, Cole re-established himself as one of England's deadliest marksmen, his goals helping United to a historic treble in 1999.
Fernando Torres (Liverpool to Chelsea)
Torres had been sensational in his three-and-a-half stint at Anfield, so Liverpool fans had reason to despair when the Spaniard joined Chelsea in a British record deal in January 2011. Chelsea chairman Bruce Buck said it was a significant day for the club in "capturing one of the best players in the world with his peak years ahead of him."
However, while Torres was only 26 when he arrived at Chelsea, there had been signs, during his final few months at Liverpool, that he wasn't quite the same player that had set the Premier League alight back in 2007. Indeed, Torres turned out to be a flop. He did help the Blues to the Champions League but he was never able to find the net with the same regularity as his early days at Liverpool. He was eventually sent on loan to AC Milan before returning to Atletico Madrid in 2015.
Samir Nasri (Arsenal to Manchester City)
Interestingly, City chased Alexis Sanchez in the summer of 2011 only to lose out to Barcelona. After that, they swooped for Samir Nasri who, at one stage, looked set to join Manchester United. However, Roberto Mancini's men stole a march and captured the French playmaker, bolstering an impressive attacking line that already included Sergio Aguero, Edin Dzeko and David Silva.
Nasri's time at City, much to the delight of Arsenal fans, was far from happy. The former Marseille man struggled to nail down a place in the starting line-up before joining Sevilla on loan. He completed a permanent move away from the Etihad Stadium last summer, joining Turkish club Antalyaspor for just €3.5 million.
Raheem Sterling (Liverpool to Manchester City)
Sterling's move to City was the most acrimonious and protracted transfer saga of 2015. The winger's relationship with the Liverpool hierarchy had deteriorated he rejected a new contract at Anfield in March before his conduct drew stinging criticism from supporters. Sterling had requested to be omitted from the Merseysiders’ squad for a two-week tour of Thailand, Malaysia and Australia and also called in sick twice, missing two days’ training, in an attempt to force through his move.
Sterling's return to Anfield was an uncomfortable one. Booed relentlessly by the Kop, he was hooked off at half-time by Manuel Pellegrini after making little impact as the Reds thumped City 3-0 in March 2016.
Emmanuel Adebayor (Arsenal to Manchester City)
Another painful transfer involving a high-profile departure from Arsenal, Emmanuel Adebayor went one further in his response to vitriol from Arsenal fans by running the length of the pitch to celebrate in front of them after scoring for City. The Togolese striker's stunning celebration unsurprisingly attracted a furious response from Gunners supporters, who hurled objects in his direction. Although he admitted to regretting that celebration, Adebayor angered Arsenal further when he joined Tottenham in 2012.
Sol Campbell (Tottenham to Arsenal)
Arsenal have lost several key players to rivals over the years but it was Wenger's men who profited from Sol Campbell's stunning decision to leave Tottenham in 2001. Campbell was a leader and a hugely influential figure at White Hart Lane, but his ambition to play in the Champions League saw him join Spurs' bitter rivals on a free transfer when his contract ended.
What embittered Spurs fans was that Campbell had previously stated he would never sign for the Gunners and also hinted that he would extend his deal to stay at the club. Then, when it was announced that he was leaving for Highbury, he was branded as a traitor and was duly met with 'Judas' chants on subsequent returns to White Hart Lane.
In terms of a career move, though, it was hard to argue with. Campbell spent five years at Arsenal, winning two Premier League titles and two FA Cups. He also scored in the Champions League final against Barcelona in his final appearance. Spurs fans have never forgiven him.
In August 2012, Alex Ferguson pulled off one of his most impressive transfer coups when he persuaded Arsene Wenger to let Robin van Persie join Manchester United. The Dutchman was insistent on joining United, leaving Wenger no choice but to let a player - fresh off scoring 30 league goals for the Gunners in the previous season - join their bitter rivals.
It proved to be a shrewd piece of business for Ferguson as van Persie scored 26 Premier League goals to clinch one final title for the Scot before his retirement.
