Revealing mistake: In the scene where Shelby goes into kidney failure and is passed out on the stairs of her home, Jackson runs over to her and grabs her up. Looking closely, you can see Shelby hold her head up. If she were really unconscious, her head would fall back as Jackson sweeps her up into his arms.
Revealing mistake: When Truvy is blow-drying Shelby's hair after she's cut it short, you can tell the blow dryer is not really turned on. The sound was obviously added in later at a lower volume so the characters could talk comfortably over it. Dolly Parton is running her fingers through Julia Roberts' hair to hide the evidence, but if you look closely, you'll see that none of the hair is actually being blown. The only hair moving at any given time is the part that Parton is actually touching.
Revealing mistake: During the Christmas scene at Truvy's, after Enelle has just finished decorating the outside of the house, she stands on the porch and talks to Shelby and Truvy. We see Clairee in the foreground, her lips moving and her animated, as if she is talking to someone just off camera. (This is a common practice in plays, for example, if we want the audience to hear one conversation, all the other characters will pantomime talking so the conversation can be heard.) So this would be fine - except that there is no one left for Clairee to be talking to. Everyone else has either gone into the house or is on the porch behind her, and Oiser hasn't arrived yet.