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.
Answer: I believe it is because the child playing Jack Jr. was basically crying through this whole scene, as you can see. It's most likely his (actually her, as the real baby was a girl) real mother holding him to calm him down. Also, when the family is saying "Blow, blow," you can hear a female voice that is neither Julia Roberts or Sally Field.
When she is sitting with Jack Jr, it is before she goes to get her hair cut. When she tells M'Lynn she wants to cut her hair, she is wearing it in a banana clip and you can see the hair past her shoulders. When "she's" sitting with Jack Jr, there's no hair to be seen. Total stand in.
Daniel Camp is the actor who played Jack Jr.
Along with Daniel two other children played Jack Jr, C. Houser and Clara Gabrielle.