Revealing mistake: When Carol goes to the dress rehearsal, she is wearing one outfit when she is talking to Mrs. Goodwyn, but then when she goes upstairs to the girl's bedroom and talks to Marcia, she is wearing a totally different outfit.
Suggested correction: Carol was talking with Ms. Goodwin during school hours, and when talking with Marcia about it in the bedroom, it was presumably evening. It's possible Carol may have taken a shower in between that time frame offscreen, thus changing her clothes.
The Big Bet - S3-E18
Revealing mistake: When Bobby closes the sliding door behind him, the curtains swing outward and through where glass should be, revealing it to be an empty frame. (00:01:15)