In the episode when Nick is "In Love" With Penny, the girl who he wrote the song for, he is tlaking to everyone for advice. There father on the show is wearing a bath robe and shower cap. He says something along the lines of how there mother was the 23rd love of his life and walking past 22 girls on the bus until he saw her. Nick laughs, but says "No not you, the book" or something like that. He stands up and his father is giving them advice. When they have the camera angled toward the father so you can see his face and the back of Nick's head, he is wearing the shower cap. When they switch the angle, And its Nick's face that is shown, you can see the back of there fathers head. The slip up: You can see his hair and his actual head and not the shower cap. when they switch back to the original with Nick's back being shown, the father is once again wearing a shower cap.