After the song "Teenage Dream" finishes, you can catch a glimpse of Damian McGinty (from The Glee Project (2011)) in the audience.
Jane Lynch recorded segments for the film as Sue Sylvester but the scenes were not used in the final cut.
Unlike most concert movies, the actors are actually in character as their Glee personae even during the backstage scenes. According to director Kevin Tancharoen, if they were playing themselves the film would have been completely disconnected from what made the show popular as the characters mean something different to everybody else.