As a Manchester United legend, pulling on a Manchester City shirt is one of the worst possible things you could do, but Peter Schmeichel has gone one step further.
Schmeichel, who spent nearly a decade at Old Trafford, has sensationally claimed City to be bigger than their Manchester rivals.
The Great Dane made the shock remarks in an interview with BBC Sport ahead of this Sunday's Manchester derby, and his views will be hard to take for United fans who idolised the goalkeeper during his spell with the club.
Following eight years under Sir Alex Ferguson, the goalkeeper left United in 1999 to spend two years in Portugal with Sporting Lisbon.
However, he surprised English football fans by returning to these shores with Aston Villa in 2001, before committing the ultimate sin and signing for Manchester City upon their Premier League return in 2002.
While United struggled to cope after Ferguson's 2013 retirement, City flourished under Manuel Pellegrini and claimed a second Premier League title in just three seasons.
Such was their difficulty in adapting to life post-Ferguson, United failed to even qualify for this season's Champions League, and Schmeichel believes recent events make City the bigger club.
The 50-year-old said: "Man City are the champions, they had their second Premier League win last season and maybe at the moment they are a bigger club than Manchester United in terms of results so the significance has changed."
Those comments will add spice to what already promised to be a heated Manchester derby on Sunday lunchtime.
Despite being five placed behind City, United could move to within a point of their rivals should they claim victory at the Etihad Stadium.
In recent years this fixture has had a huge impact on the outcome of the Premier League title race, adding a significance to the rivalry that Schmeichel never witnessed.
"In my first couple of years (at United) I didn't understand (the rivalry with City). Liverpool and Leeds were the much bigger," said the ex-goalkeeper.
It's easy to see why the Danish international, who never lost a derby during his United days, failed to consider City as a main rival during his first spell in Manchester.
While he was busy picking up five Premier League titles and becoming a treble winner, City dropped from the top flight to the third tier of English football.