Transfers can be good, bad and ugly. Sometimes they can be really ugly. Here are 10 of the most controversial transfers in the history of the game.

10. John Obi Mikel, Man United to Chelsea

"It was really confusing," Mikel said recently about the saga of his transfer to Chelsea. "And I was a kid, you know?" The Nigeria midfielder was a kid, only 19 and playing for Lyn Oslo, when Manchester United announced they had an agreement to sign him in January 2005, only for Chelsea to claim they had one too. There followed a rather unseemly row in which FIFA got involved, only for them to ultimately tell Mikel he had to decide. He chose Chelsea, who then had to pay United £12 million for a player they never actually signed. A decent deal for United, all told.

9. Carlos Tevez, Man United to Manchester City

A transfer rooted in the murkiness of third-party ownership. Tevez had technically been "on loan" to Manchester United from Media Sports Investment, agent Kia Joorabchian's company, for two years between 2007-2009. United allegedly told Tevez they were going to sign him permanently, but the deal never happened and City swooped. Of course, this being the very noisy City, they made a noise about it, putting up billboards (strategically positioned to face toward Salford) with Tevez's picture and "Welcome To Manchester" emblazoned on it. Safe to say it caused some irritation, and United boss Sir Alex Ferguson in particular was irked, saying the poster was was "stupid and arrogant" and that City were "a small club with a small mentality."

8. John Robertson, Nottingham Forest to Derby County

This was the transfer that ended one of the greatest relationships in English football history. In 1982, Peter Taylor left Brian Clough's side at Nottingham Forest to manage their arch-rivals Derby County, where the pair had won their first league title, but if that wasn't enough, a year later he returned to his old club to sign Robertson, probably the greatest player in Forest's history, but who was on the wane by that point. The problem was he didn't tell Clough, who was on a charity walk when he found out about the transfer. Clough never forgave his old friend, famously and callously saying that if he saw Taylor's car had broken down by the side of the road, he would run him over. The pair never spoke again, something Clough said he regretted when Taylor died in 1990.

Robin van Persie's move to Manchester United from Arsenal was devastating to Gunners fans. Tom Purslow/Manchester United via Getty Images

7. Robin van Persie, Arsenal to Manchester United

By 2012, Arsenal were used to their star players leaving for better prospects of winning silverware. From Patrick Vieira and Thierry Henry, through to Samir Nasri and Emmanuel Adebayor, each departure ate away at their soul a little, but Van Persie's £24m departure to Manchester United ripped out their heart. As much as anything, it was yet another sign that the two clubs were simply not rivals anymore, and United were able to essentially take who they wanted. "I always listen to the little boy inside of me in these situations," Van Persie said of his decision to choose United. "That boy was screaming for Man United." And that boy won the Premier League title in his first season at Old Trafford with 26 goals in 38 games.

6. Ashley Cole, Arsenal to Chelsea

"Tapping up" is one of those things that is technically not allowed in football but not a crime that is usually prosecuted, though it made headline news in January 2005 when Chelsea approached then Arsenal left-back Cole about moving across town. Cole, disgruntled with discussions over a new contract at Arsenal, met Blues boss Jose Mourinho in a hotel restaurant, when he was very much still a Gunner, a meeting that was declared an illegal approach by the Premier League. Cole was fined £1000,000, reduced to £75,000 appeal, Mourinho £200,000 and Chelsea £300,000. None of which prevented Cole from moving to Stamford Bridge the following year in a swap deal with William Gallas, to much acrimony.

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5. Johan Cruyff, Ajax to Feyenoord

It takes guts to make a move out of spite and know it would work. Cruyff had that courage. In 1983, the 36-year-old club legend had just reached the end of the second year of his second spell at Ajax, but the board seemingly thought he was finished, overweight and over the hill. Thus they didn't offer him a new contract, so a furious Cruyff signed for arch-rivals Feyenoord, where he won a double and was named Dutch footballer of the year. "I wanted to take my anger out on Ajax via Feyenoord after the club had thrown me out with the rubbish," he later wrote in his autobiography.

4. Roberto Baggio, Fiorentina to Juventus

Juventus vs. Fiorentina is one of Italian football's slightly lesser known rivalries, but the enmity runs deep. So when Fiorentina sold Baggio for a world record fee of £8m in 1990, it didn't go down well. Viola fans laid siege to the club's offices, president Flavio Pontello was forced to take refuge in their stadium, and the subsequent riots led to 50 injuries and nine arrests. When the two sides played each other the following season, Baggio refused to take a penalty, ostensibly because he said Fiorentina goalkeeper Gianmatteo Mareggini knew him too well -- though Luigi De Agostini missed anyway and Juve lost 1-0. The real issue was that, on his way off the pitch after being substituted, he picked up a Fiorentina scarf thrown from the crowd, later saying his "heart was purple." A reported 300 Bianconeri fans showed up to training a few days after to register their disapproval.

3. Sol Campbell, Tottenham to Arsenal

A theme in a lot of the transfers on this list is broken promises. Back in the 2000-01 season, everyone knew that Campbell's contract at Tottenham was running out and he was considering his options, but few thought he was even going to leave the club he had spent 12 years at, never mind move to hated rivals Arsenal. That was largely because Campbell had stated that he wanted to stay then, after changing his tune and indicating he wanted to leave for Champions League football, insisted he wouldn't join Arsenal. It was hardly a surprise that when he did cross North London on a free transfer, the Tottenham Hotspur Supporters' Trust called it "the ultimate act of betrayal."