When Bob, Linda, Louise, and Gene discuss Tina getting detention, Gene is laying face down on his parents' bed kicking a pillow with his feet. The pillow underneath his feet temporarily vanishes but indentation marks continue to form at the end of his shoes as he kicks, as though the pillow were still there.
At around 4 minutes in, Bob, Linda, Gene and Louise are in the parent's bedroom discussing how bad Tina has gotten. You can see Gene lying on his stomach on the bed. In one shot the pillow that he is kicking disappears. The crease lines are still there floating in mid-air but there is no pillow underneath, you can see straight through to the bed.