In Arsene Wenger's 20 years as boss of Arsenal he has had feuds with various managers.
Despite numerous clashes with Sir Alex Ferguson, arguments were forgotten and only last season Fergie offered his support to the pressured Arsenal manager.
Between Wenger and Jose Mourinho though, things have been said. Bad things.
Mourinho has called Wenger "a specialist in failure", a "voyeur" and publicly criticised Wenger's win percentage in the Premier League. Meanwhile Wenger has said Mourinho is "disconnected with reality", "disrespectful" and "always" complaining.
Where this footballing feud will go from here is anybody's guess.