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).
Millie writes "Things to say to Sam on our Date!" at the top of her notepad at 14:20, but at 14:29 that has been underlined and embellished without her having the time to do that.
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 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 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.
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.