When Beverly goes to Chip's school for a P.T.A. meeting, Mr. Stubbins says Chip is "off to a fine start this year". However, a card in the opening scene established that the day is Friday, May 14, 1993, which would be nearing the end a school year, not the beginning.
In the first Sutphin family dinner scene, Beverly (Kathleen Turner) serves Misty (Ricki Lake) a piece of meatloaf. In a medium shot of Beverly, the piece of meatloaf is clearly the end or "heel" of the loaf, dark and rounded on one side. In the next shot, a medium shot of Misty, the piece of meatloaf is from the middle of the loaf, a lighter shade, large, and not rounded on one side.
In the main-title sequence, when Beverly swats a fly on a tray on the Sutphins' kitchen table, she brings back the swatter to an upright position as quickly as she swats the fly. However, in closeup in the next shot, the swatter is down flat on the tray. Once established, the swatter is pulled away more slowly than in the preceding shot, revealing the squashed fly. After the squashed fly is revealed, the director's credit appears.
When Mrs. Sutphin arrives at her son's school to meet his teacher, the time is given as 3:36 pm. Very soon after, she takes her seat in the hall and is invited almost immediately into the classroom, where the clock reads 4:10 pm.
The goof items below may give away important plot points.
When Beverly is grilling Mrs. Ackerman in court, she asks her if the scissors that were found in Mrs. Sterner's stomach were hers. The scissors wouldn't have been found in her stomach, as Beverly removed them and threw them at Mr. Sterner, missing him and lodging in the hallway wall, where they would have been found.
Beverly is so surprised by her lawyer's opening statement calling her not guilty by reason of insanity that she fires him and represents herself. The judge then asks for her plea. By this point her plea would have already been entered and as the defendant Beverly would of course know what that plea is.
The date for the trial is October and the establishing shot of the courthouse shows fallen leaves on the grass. In the next shot, the Sutphin family arrives and there are green leafy trees behind them.