Both Catalans and Merengues, including some of the biggest names in world football, have committed this perceived sin over the years and all have served to stoke the fires of what is one of the most fierce and bitter rivalries in world football.
The Cameroon international arrived at Real Madrid in 1996 and made his first-team debut two years later. But he was limited to just a handful of appearances and left the Bernabeu in 2000 for Mallorca, where he rose to prominence. Then came the big move to Barca, where he spent five seasons making his name as one of the continent's most prolific strikers and one of Africa's greatest-ever players.
The German played most of his career in the Spanish league, signing for Barcelona from Cologne in 1980. But his relationships with a string of the club's managers, including Terry Venables, were strained and in 1988 he swapped the Camp Nou directly for the Bernabeu. He spent two successful years in the Spanish capital before moving across town to Atletico.
The Argentina international arrived in Barcelona as one of the most exciting young prospects in South American football but, despite a decent scoring record, he was deemed surplus to requirements and was offloaded on loan to Monaco then Sevilla. Once his contract expired at Camp Nou, he was shipped off to Madrid, where he failed to make an impact and two years later he quit Spain for Benfica.
A product of Barca's youth system, Celades made his debut during the 1995-96 season and eventually became an important member of the side which rose back to domestic supremacy in the late 1990s. After a stint with Celta Vigo, he signed for Real in 2000, although he was used sparingly at the Bernabeu and, after a loan move to Bordeaux, eventually moved on to Real Zaragoza.
The biggest traitor of them all, if the reaction to his move from Barcelona to Real is anything to be judged by. The Portuguese became one of Barca's best-loved players having arrived in Spain in 1995 and spent a hugely successful period at Camp Nou. But when he decided to become a pawn in Florentino Perez's presidential campaign in 2000, swapping blue and maroon for white in a bumper 60 million euro deal, he rapidly turned from hero to villain. The vitriol directed at him by his former worshippers came to a head, quite literally, when he returned to Camp Nou in 2002 - a pig's head was thrown in his direction from the stands, among whisky bottles and other items.
The Danish wizard signed for Johan Cruyff's Barcelona in 1989 and quickly became one of the leaders of the Dutchman's dream team, winning four Liga titles on the bounce. But strained relations between him and Cruyff eventually led to his departure, with Madrid the benefactors. Laudrup won a fifth consecutive title with Real before quitting Europe for Japan a season later.
Ronaldo arrived at Camp Nou in 1997 and 47 goals in 49 appearances later, he had become one of the most sought-after players in world football. But the Brazilian was not entirely happy and after failing to reach agreement on a new deal, he bought out his contract and left for Internazionale in Italy. He returned to Spain five years later - with Real. In the white of los Merengues, he scored 23 goals on the way to the league title in his first season, but failed to add to that silverware in the remainder of his five years there.
Striker Perez played five mediocre seasons with Madrid before moving to Real Betis and then on to Barca in 2000. He managed just two goals in his debut season at Camp Nou and was eventually loaned to Marseille, where he was equally ineffective.
The stylish Croatian spent three seasons with Real Madrid from 1991 to 1994 and after a slow start, he became a fixture in their midfield. Having won two runners-up medals in the league, Prosinecki moved to Real Oviedo for a season before jumping ship to Barca. His stint at Camp Nou was relatively unremarkable though as injuries hampered him although he did manage to win another runners-up medal before moving on to Sevilla.
The Romanian joined Madrid in 1990 off the back of his performances at the World Cup in Italy, and he quickly demonstrated just why he had the nickname 'the Maradona of the Carpathians'. A two-year stint with Brescia came before he moved back to Spain with Barca, where he played less, but still lived up to his reputation as one of Europe's best attacking midfielders.
The versatile attacking midfielder spent five seasons at Real Madrid before seeing out his contract and moving to Barca for free. The move enraged Madrid fans and he instantly became a hate figure in the capital where feelings were not helped by his eventual captaining of Barcelona. He spent eight years at Camp Nou before retiring and moving into management with Barca's B team. He now coaches Roma.
Real Madrid's current coach cut his teeth at Barca, where he learnt his trade from the late, great Bobby Robson and Louis van Gaal, winning the European Cup Winners' Cup and two Liga titles before moving into full-blown management at Benfica. Uniao de Leiria, Porto, Chelsea and Internazionale followed before Mourinho moved to take up the reins at Real Madrid, where he is now tasked with getting one over his former employers.