They are best known for being key players for some of the best teams in the world – but did you know they also have a past with their bitter rivals?
1. Jason Leonard (Saracens)
The prop grew up supporting Sarries, but following just a year with the black and reds, he moved to local rivals Harlequins, where he stayed until his professional retirement in May 2004, making a total of 290 appearances for the team. He also became an England and Lions legend.