In Pop Quiz when Cory and Shawn are talking in Cory's room about copying the summary off the video for their book report, there is a ball on Eric's bed. Then it disappears, then it is back again.
When Cory and Shawn come home from school, Mr. Feeny tells them to read a book. Cory says if he makes the basket he will read. He clearly misses and the ball goes out the window. In the next shot it shows the yellow ball sitting on the bed, then it shows just Shawn, then in the next shot again the ball is missing.