Continuity mistake: The Christmas decorations in the outside of the house change as to where they are and what they are.
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.
Continuity mistake: In the wedding reception dance scene, you see a couple dancing amongst the guests on the dance floor. The female is wearing a seafoam green dress with a large matching bow in her hair. About 2/3 of the way through the song "On the Bayou," you see this same couple run onto the dance floor from off camera to watch the featured dance couple, as if they had never been on the dance floor at all.
Continuity mistake: Janice van Meter's dress at beginning of the reception is floral with a flowing bottom. Then when when dancing with Trudy's son and looks like "2 pigs fighting" its tight and cream.
Continuity mistake: When M'Lynn and Truvy are sitting in the backyard drinking tea after the funeral, there is no greenery next to where Truvy is sitting. In the next shot, when Clairee and Ouiser walk in the backyard together next to Truvy, there are now bushes and some trees that weren't present in the shot beforehand.
Continuity mistake: In the scene after Shelby's funeral has taken Place where M'Lynn, Truvy and baby Jackson are in the backyard, Annelle is bringing the two ladies some tea. In the next shot, Ouiser and Clairee walk into the backyard and now present in the shot next to Truvy is a small tree and shrubbery that was not there before.