It takes a certain type of character who is willing to move from one club directly to their local rivals.
In fact, no player is probably more suited to pulling off such a stunt than Carlos Tevez.
The Argentine maverick made the risky switch from Manchester United to Manchester City in 2009 after two successful seasons at Old Trafford.
Article continues below
Tevez soon became public enemy number one for Red Devils fans as his decision was judged as a betrayal to the team that had turned him into one of the game's big stars.
It was also difficult for some of his former Man United teammates to handle - particularly Patrice Evra.
Article continues below
The former Man United captain has recalled how he felt after hearing 'Carlito' had joined the 'noisy neighbours'.
“Carlos Tevez was my close friend. But I was not pleased when he joined Manchester City in 2009. I was really annoyed because he was a great player," Evra told the official United website.
"There are two sides to every story, I understand that and football can be complicated, but I didn’t like it when he was on the banner saying ‘Welcome to Manchester’ after he signed for City and I told him so. ‘Carlito!’ I said. ‘What are you doing? This shirt isn’t you! I have been to your house and you still have a picture of yourself wearing a United shirt near your pool table! You are a Red. You know deep in your heart that you are a Red.’”
As admirable as it was, Evra's attempt to persuade Tevez against the move proved ineffective and the Argentine turned into a thorn in Sir Alex Ferguson's side.
None more so than the brace he scored against his old side in a Carling Cup semi-final, just a few months after swapping allegiances.
And Evra added he remained good friends with the now Boca Juniors forward - something Ferguson felt uneasy about.
He continued: “We joked a little, we stayed close friends and Mr Ferguson didn’t always like that, especially when he saw us pictured outside a restaurant in a newspaper.
"He didn’t want his players associating with those from City, who were becoming a much stronger team, but I am not just a footballer, I am a man who is loyal to his friends and Carlito was my friend.”
But the French full-back kept in touch with Tevez and felt that he still had 'affection' for the Red Devils, despite his acrimonious exit.
“‘Listen, Carlito,’ I said. ‘We are friends but don’t try and be nice with me. Don’t shake my hand and think that everything will be fine between us.’ He was laughing. He said that City were going to knock United out, but deep down I think he still had a great affection for United.
"He once told me that he would always respect United fans. I know he did well for City, I know he was a great player for them, but he’d won Premier League titles and the Champions League with United, he’d played in the best team in the world. He couldn’t just forget all that.’”