Continuity mistake: When Kesha has posted the video of herself telling viewers that her competition is back on, the bar showing how long is left of the video moves backwards in each shot.
Audio problem: When Tori wins the Kesha contest, when Kesha is singing, you can tell she's lip syncing because when she says "this place about to blow," she is just moving her lips, not singing.
Other mistake: When the gang is trying to find the letters to spell Ke$ha at Tori's house, Robbie decides to go give the extra ice cream to the kids at the park. When he comes back he says that there were kids there with their moms, despite the fact that it is late at night, as we can see out of the window.
Answer: The Director hated working with Melinda, but knew that her superstar popularity and status would get his movie a big buzz and higher ratings. And the movie was discontinued for a re-audition to fill-in Melinda's leading/major role.