#1. The Undertaker - Confronted CM Punk over his casual attire
While The Undertaker was feuding with CM Punk on WWE TV, they had a legitimate issue with each other backstage. Or rather, The Undertaker took an issue with then-World Heavyweight Champion CM Punk.
The Undertaker reportedly confronted Punk about his "casual attire" as the World Heavyweight Champion, concerned over how it would affect the company's image. It's important to note that by this point, The Undertaker was a long-time and well-respected locker room leader.
CM Punk responded by arguing that John Cena, the face of WWE at the time, dressed in a casual attire too. This remark didn't sit well with The Undertaker and WWE higher-ups.
Dave Meltzer of the Wrestling Observer Newsletter noted that this disagreement ultimately led to the World Heavyweight Championship changing hands at Hell in a Cell:
"This got back to WWE management and they took it as Punk thinking he was a bigger star than John Cena. Then the decision was made to take the belt off Punk at Hell in a Cell and put the match as the opener. There are a couple versions of the story going around, but which ever one is being told, the end result remains the same." (via SEScoops)
Jim Ross, a renowned WWE broadcaster at the time, later denied these claims, though. He said that CM Punk was a "different" type of talent to work with and not everyone understood how to get along with him. He also confirmed that the WWE office didn't back CM Punk the way they should have and described it as "sad".
CM Punk ultimately faced The Undertaker a few years later at WrestleMania 29, where The Phenom earned the final win of his incredible undefeated streak at WrestleMania.