When Donald goes through the windshield, you hear the sound of regular glass breaking. You can also see a pane of glass break off the car. Real windshields don't break like normal glass does. It doesn't make a shattering noise and doesn't shatter like window glass.
Donald suggests Flowers for Algernon as an example of a movie about flowers, to which Charlie replies, "It's not about flowers; besides it's not a movie." In fact, the book had been filmed twice - once as Charly and for television as Flowers for Algernon.
When Charlie is getting take out from the restaurant and bumps into Valerie, he leaves the restaurant in a hurry and does not take his bag of food with him. In the next scene, we see him eating the food.
When the twins talk about the serial killer in the script of "The 3" being a literature professor, Charles' hands are clasped. In the close shot of Donald with Charles in the foreground out of focus, his arms are folded.
In the swamp scene when Susan Orlean is cradling John LaRoche's head, the hair on his forehead is totally disheveled. Then in a closeup, his hair sweeps neatly across his forehead. Back to the wider shot, his hair is disheveled again.
Immediately before Charlie has a tantrum in the New York hotel room, the lamp to the right of the bed can be seen leaning against the wall in the corner, but it is standing upright before he knocks it over in the next shot.
In the beginning of the film, when John is trudging through the swamp searching for orchids, he is seen up to his armpits in water. Once out of the water, when the park enforcement officer approaches them, the water line on LaRoche's shirt is clearly seen as just above the waist - about a foot lower than the point to which he was submerged.