When George is introduced to all the guys in the station wagon, you can see his present to Sam reflected in the side window behind him (at around 37 mins), although George is holding the present in his hand. The commentary notes the reflection is from a second prop gift in the storage area behind the rear seat that they didn't realize was reflected in the window.
When George is asking Sam if the plan to take him out on the boat was true you can see George in the background moving his lips as if he is speaking but there is no audio to match this (at around 2 mins). This was corrected in the DVD.
When George is lying on the shore and Clyde runs to shout for help, he stumbles and falls almost entirely into the water (at around 47 mins). A few shots later he stands facing the others and is seemingly dry (at 53:04 to 53:07 and 53:23 to 53:27), definitely too dry to have just been immersed in water.
When dropping Millie off at her house after returning from their misadventure, Millie exits the car with no socks and she is wearing her tennis shoes like clogs with her heels exposed (at around 1h 09 mins). When she enters her house and climbs the stairs, her socks are on and shoes cover her entire feet (at around 1h 09 mins).
When Sam is talking to Rocky about calling off the prank they planned to play on George, a utility boat can be seen in the background, to the left (from 26:59 to 27:05), even though they are supposed to be on the river alone.
The angle of their parked car with respect the barn in the morning, pointing toward the near corner of the barn (at around 1 min), is quite different later in the day, when the kids return from the river and it has turned parallel to the end of the barn (at around 7 mins).
When Sam is talking to Rocky about calling off the prank, the position of his hands on the oar he is holding keeps on changing, and in some close-ups his hands and the oar are not visible, when they should be (from 27:48 to 28:08).
When the group arrive at the boat, Marty throws his oar into the boat, and then takes a beer out of his case (at around 23 mins). Then in the next shot (at around 24 mins), the beer and case have vanished and now he is lighting up two cigarettes instead.
When Marty proposes the truth or dare, he throws away his cigarette (at around 16 mins). But moments later (when George insults Clyde, Sam and Millie at 40:46) he's holding a cigarette in his hand, yet we never see him light one.
When George is getting into the car with all the boys to go to the lake, he is wearing a gold chain (at around 18 mins), but once in the car, the chain is no longer around his neck. It could actually be hidden by his orange shirt and white thermal undershirt which fit more closely around his neck in some shots than others -- it is not visible from 17:28 to 17:34 but is visible again at 17:38.
When Millie is writing what to say to Sam on a first date, what she writes differs slightly from what she reads out loud. Whereas she says 'So, Sam, it's a beautiful day' as she writes the first item, when it cuts to the shot of the paper she's writing on (at around 29 mins), she's writing 'So, Sam, it's a really beautiful day'.
When Millie is shown writing in her diary, she switches from being right-handed (from 14:14 to 14:35) to left-handed (from 14:36 to 14:44). The left-hand scene is Millie in the MIRROR! Her gold bracelet is on her right wrist, the hand she writes with. In the mirror, it appears to be her left. Also, the table where she grabbed the pen with her toes was on HER right side. But in the mirror it looks like it was HER left side.
In the scene where Marty and Rocky meet Kile and Jasper who have been eating mini-beef tacos in the Quickie-Mart, Jasper is eating from a yellow mustard packet in his left hand (at around 1 min), then a green relish packet in his right hand (at around 5 mins), then a yellow mustard packet in his left hand (at around 8 mins), then a green relish packet in his right hand (at around 13 mins), and then a yellow mustard packet in his left hand (at around 28 mins). Also, the paper food-boat in his other hand switches correspondingly from right to left to right to left to right.
The goof items below may give away important plot points.
After Millie refuses to cut her index finger (at around 1h 03 mins), you can see yellow tape tied around the trunk of a tree in the background as Marty gets up (at around 1h 03 mins). At the end of the film police have cordoned off an area with yellow tape where the body is buried.
After Marty proposes that they bury George's body, Clyde, facing the camera in the foreground of a medium wide shot, turns and starts to walk away while Millie's arms are at her sides (from 59:13 to 59:16). The scene cuts to a closer shot of Millie and Sam with Clyde walking away in the background while Millie is in the foreground with her arms crossed at her chest (from 59:17 to 59:20). In the next shot (from 59:20 to 59:30) Clyde is in the foreground and Millie is in the background with her arms at her sides.