Robin Van Persie's hat-trick led to plenty of glum faces in Southampton this weekend, but there can't have been too many happy ones in north London either.
Arsenal fans will have watched their former-captain with that familiar feeling as he put on a masterclass in forward play. Many expected the Dutchman would need at least a modicum of time to settle into his new surroundings, but the change of shirt has not been followed by a change in performance.
Four goals in two league starts means Old Trafford has a new hero - his past as an icon in the red and white of north London all but forgotten.
Similarly, Andy Carroll's escape from Merseyside was greeted with a degree of relief in Liverpool. The £35m striker had become a burden for the Reds, a lightning rod for criticism of the old regime, and a painful reminder of the wasted millions of the Kenny Dalglish era.
But, in West Ham colours, the England striker showed what he's capable of when deployed in system designed to accentuate his strengths.
But Arsenal and Liverpool, are the not the first, and will not be the last, to potentially rue the sale of a star asset.
Over the years, there have been plenty of examples. GMF looks back at some of the best…
Eric Cantona - Leeds to Manchester United (£1.2m - 1992)
Cantona spent less than a year at Elland Road, and Leeds surely regret not holding onto the French international.
The Yorkshire club made just £300,000 on his transfer to United, and Sir Alex Ferguson got a player who would go on to become one of the most iconic stars of the Premier League era.
Xabi Alonso - Liverpool to Real Madrid (£30m - 2009)
The sale of Fernando Torres and Xabi Alonso coincided with a brutal collapse in Liverpool's fortunes, from a 2nd place finish in 2009 to last season's eighth-placed disappointment. But Alonso was the sale most Reds fans now regret.
Reportedly the Spaniard never really wanted to leave Anfield, and has even revealed he could return to finish his career back on Merseyside. But his departure was a huge blow for the club - Steven Gerrard said he was left "devastated" by the transfer.
Cesc Fabregas - Barcelona to Arsenal (free - 2003)
Fabregas moved to Arsenal as a 16-year-old, sensing that his first-team opportunities would be limited at the Nou Camp.
And the Spaniard flourished in north London, making his debut a few months after signing, and was named captain aged 21.
Barcelona recognised their mistake, and did everything they could to sign him back, eventually correcting their error by resigning Fabregas - at the expense of €35m. A costly repatriation.
Frank Lampard - West Ham to Chelsea (£11m - 2001)
The Hammers looked have a secured a decent deal when they flogged Lampard to Chelsea, but the Blues midfielder has since gone on to become one of the best midfielders in Premier League history, while West Ham have flitted between Premier League and Championship.
That £11m fee now looks like fantastic business as Lampard has gone on to become the league's highest scoring midfielders, consistently firing 20 goals a season, and establishing himself as a key England regular for much of the remaining decade.
Vincent Kompany - Hamburg to Manchester City (£6m - 2008)
Despite the hundreds of millions spent since Sheikh Mansour's arrival, Manchester City's best signing remains the £6m sent to Hamburg for Vincent Kompany.
The City captain has become the defensive lynchpin of a Premier League winning team, and brings a presence to the side that could otherwise be lacking in a club so rapidly assembled.
But Hamburg will not remember the Belgian too fondly, having signed him for close to £7.5m in 2006. Two years, and just 29 league appearances later, and Kompany was off to City for £6m.
Honourable mention | David Ginola - Newcastle to Tottenham Hotspur (£2.5m - 1997)
Ginola enjoyed two fantastic seasons at St James' Park, finishing second twice in the Premier League under Kevin Keegan. The Frenchman was a massive fan favourite on Tyneside but fell out of favour once Kenny Dalglish replaced Keegan, and moved to Spurs for £2.5m - the same price Newcastle paid Paris St Germain for his services in 1995.
Ginola had been linked with Barcelona and Real Madrid, so it was quite a coup for Gerry Francis to bring him to north London.
And the flamboyant winger was a huge success at Spurs, winning the PFA Players' Player of the Year, the FWA Footballer of the Year and the League Cup in 1999.
Despite spending just three years at White Hart Lane, the Frenchman was inducted into the Tottenham Hotspur Hall of Fame in 2008, while Newcastle would never scaled such heights in the Premier League.