This slip up occurs in season 3, episode 13, titled "Home But Not Alone".

Check out the scene when Melinda is bringing what looks like groceries to the family. She walks into the kitchen and the little brother is sitting at a table looking at some sort of memory book. As soon as Melinda walks in, he closes it and you can clearly see that it has an orange cover with the word "dad" on it. The book is vertical to him and it's near him. Melinda then asks what he's looking at and when the camera pans back on the child, the book is now horizontal, face down, and away from him.