Arsene Wenger admits he is still haunted by the decision to sell "special" Nicolas Anelka to Real Madrid just two-and-a-half years into his Arsenal career.
Anelka established himself as one of Europe's most-promising young strikers after arriving from Paris Saint-Germain as a 17-year-old in February 1997.
The Frenchman won the Premier League - FA Cup double during his first full season and was crowned PFA Young Player of the Year for his 17-goal return the following campaign.
However, Anelka endured a tricky relationship with the Arsenal fanbase and said he "can't stand the English press" in May 1999.
Three months later, he was unveiled as a Real Madrid player after the Spanish giants agreed a fee in excess of £22million with the Gunners.
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Nicolas Anelka profile: Inside his turbulent career, Arsenal regrets and Real Madrid misery
The north London club made vast profit on Anelka having purchased him from PSG for just £500,000, but Wenger confesses the transfer to Los Blancos still eats away at him.
Speaking at a charity event alongside former Arsenal co-owner and vice-chairman David Dein, Wenger said: "My biggest regret and shame is that Anelka left. He could have been special, instead he played for 13 clubs."
Anelka lasted just one season with Real Madrid, although he did win the Champions League, before returning to PSG in July 2000.
His winding career would take him to Liverpool on loan and to Premier League rivals Manchester City, Bolton Wanderers, Chelsea and West Bromwich Albion on permanent transfers.
Outside of England, the Euro 2000 winner also represented Fenerbahce, Shanghai Shenhua, Juventus and Mumbai City before hanging up his boots in 2015.
While Anelka scored 224 goals for club and country across his two-decade career, there is a sense he never lived up to the vast potential he showcased during his brief spell at Highbury.
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His enduring affection for Arsenal was clear to see when he did not celebrate scoring for Chelsea in their 4-1 victory over his former club in May 2009, despite Gunners fans dubbing him 'Le Sulk' due to a perceived poor work rate.
"I still love the club, the people, even if they think I don't love them," Anelka told Chelsea's official website at the time. "It is normal I don't celebrate."
The Paris native won the Premier League Golden Boot that season with 19 top-flight goals, the most he returned in a single campaign for any of his clubs.