When Fritzi insults the fat girl they are discussing casting of "Promises, Promises", but that had already been settled earlier. The same problem comes up during what appears to be auditions for "Follies", which was also already settled (during the cast sheet montage, we see that the role of Carlotta, who they are singing for, has already been cast).
While singing "The Ladies Who Lunch", Jill vomits several times onto the stage (we hear the splat when it hits the floor). After Fritzi replaces her (yanking her offstage from the wings), we see the stage and it's clean.
When Jill is in dressing room preparing for her "The Ladies Who Lunch" number, neither her wig styling or costume matches what she is wearing when she appears onstage moments later.
Following the cutting of Jenna's jaw wires, she should still have had the anchors for them on her teeth. Yet when she sings there are no anchors.
During Ladies Who Lunch, after Fritzi drags Jill offstage, she is holding a glass, filled with liquid. However, when she breaks the glass, it's empty.
After having her jaw wired shut for the whole summer, the muscles in Jenna's jaw would have atrophied to the point that she would not be able to open her mouth wide enough to sing well.
Buddy the sports instructor throws the basketball at Patrick and it hits Patrick's right shoulder, but Patrick grabs his left shoulder and says "Ow!".