Ariana's most famous feud has to be her tense relationship with her former Sam & Cat costar Jennette McCurdy. The two starred in Nickelodeon's wildly popular iCarly and Victorious spinoff, but rumors of behind-the-scenes drama plagued the show during its first season, and it was eventually sent on "hiatus" before wrapping up its first 40-episode run. The beef reportedly started due to the fact that Jennette was upset that Ariana was getting paid more than her. (To be fair, Ariana was and still is the bigger star.) That, coupled with Ariana's then-blossoming music career, led to fighting and a lot of thinly veiled subtweets from Jennette about Ariana.
While there is some undeniable drama between Jennette and Ariana, placing the blame solely on Ariana would be unfair, especially when you look at Jennette's really unprofessional behavior after the show was canceled. (Jennette made a web series, What's Next For Sarah, with an annoying character called "Gloriana.") Ariana shouldn't have to apologize for her career success or her higher salary — she was a major part of the draw for Sam & Cat, and as in most professions, bosses tend to adjust salaries according to how much of an asset they think their worker is to their team. In Hollywood, being a recognizable name is a big asset.