Every now and again - but not often - a player will make the brave decision to move from his current club to their biggest rivals.
Becoming an enemy in one community usually sees them become more of a hero in their new surroundings, but there is frequently more drama than meets the eye.
From pigs heads, controversial billboards and surprise press conferences, these deals can bring out the best and worst in football and its fans.
Here, Sportsmail takes a look at 10 transfers between rival clubs that have caused plenty of drama over the years.