During the first dinner scene, in the wide shots of the Klump family, Grandma Klump has a coke to drink instead of water. This happens twice.
The writing on the blackboard.
The amount of collard greens changes between shots on Papa Klump's plate after he pours the gravy on them.
Sherman is seen getting ready to close the microwave twice after he puts the TV dinner in.
When Buddy and Miss Purdy go to Scream for the first time, it appears that their table is almost directly opposite the front of the stage. When Buddy later plays the piano on stage and sings to Miss Purdy, he is facing to the left of stage (ie. not in her direction). If he wanted to face her while playing the piano, his head should be turned to his right rather than looking straight ahead.
Jason appears to be able to open the passenger door from the outside of Buddy Love's Dodge Viper to get in. First generation Vipers never came equipped with exterior door handles.
Klump returns home after getting a date with Carla and puts a frozen dinner in the microwave. The frozen dinner has an aluminum foil cover that Klump does not remove, but there is no evidence of the sparks that would result.
Stunt-double for Buddy Love when he throws Reggie into the piano.
When Sherman first pours the weight-loss potion into the vial, the blue lights on the vial that make the potion glow are visible. They are still lit up even after Sherman drinks the potion.
By the 1996 setting of the film, only the lowest budget TV dinners had aluminum foil covering the meal. Plastic wrap had replaced it and even the food trays themselves were mostly hi-temp cardboard or corrugated paper.
Drummer clearly not playing along to music at the ball near the end.
At the end of the first family dinner scene, the camera cuts back and you can hear Ernie Jr. continuing to laugh loudly, but it is obvious by looking at him that he is not laughing.
At the end of the first family dinner scene, Ernie Sr. looks up and yells to his father, "Damn, daddy, you rotten!" His mouth does not move.
When Carla runs out of the tent after the professor, the battery pack for her mic slides down her leg.
During the first dinner scene when Ernie Klimt Jr begins to choke on the food, you can see the shadow of the boom mic over his left shoulder.
Sherman never takes the aluminum wrap off the TV dinner before he turns on the microwave this would cause the foil to ignite and start a fire so Sherman would have likely burned the house down.
When The Professor is encouraging Carla to go on a date with Buddy Love, he ends the conversation by saying, "I'll even put in a good word with you for Buddy", a completely meaningless line. What he meant to say was, "I'll even put in a good word with Buddy for you". It is inexplicable how either nobody noticed the mistake, or they felt it wasn't worth shooting even that one single line again, at the very end of the conversation, to correct the mistake, something it would have taken less than a minute to do.