When Karen and Conrad at the restaurant, the straw in Karen hands starts unwrapped, then becomes wrapped, and then suddenly jumps into the Coke.
The boat Conrad and Buck are in is obviously stationary, and is being rocked rapidly back and forth, inconsistent with the way a vessel that size would behave during a real squall.
The golf scene is set on the 18th hole of the golf course but they are seen leaving the practice green (multiple holes in the putting surface are visible).
When Conrad enters the elevator to Dr. Berger's office for the first time, his hair changes length and style between shots.
When Karen and Conrad are at the restaurant and Karen is leaving, the glass of Coke that Conrad ordered but never touched has disappeared from where it was on the table.
At the beginning of the scene at the golf club, Calvin remarks how strange it is that there are no hills in Houston. Chicago and its surrounding area are extremely flat, so it's unlikely that a resident of that area would find lack of hills remarkable. Also, in the background in this scene are some significant hills that seem more scenic than any horizon one might have in Lake Forest.
When Conrad lies down on his bed before calling Jeannine, there is a bulletin board behind his head, on the wall above the bed. The bulletin board is missing in all other bedroom scenes, both before and after this scene. Also, taking these scenes at face value, Conrad's bedroom is maybe 5 or 6 yards wide with windows on opposite walls. None of the front or back exterior shots of the house show a space that could include this room configuration.
While Conrad is watching the swim meet and fighting outside of school, he has a red plaid shirt under a gray sweater. When he arrives back at his grandmother's house and the subsequent scenes, he has a white with thin blue stripes shirt and a beige sweater. This second outfit is what he wore for his date with Jannine, so the sequence at his grandmother's may originally have been intended to occur in the film between the date and the swim meet but later was moved to occur after the swim meet.
Before and after Beth gives candy to the trick-or-treaters, the reflection of a moving boom mic is visible in the glass covering the face of the clock near the door.