Audio problem: When Rudy and her friends make a recording of spooky noises, then play it back while trying to scare Cliff, the noises on the tape go on much longer than when the kids recorded it.
Continuity mistake: There are two jackets hanging (one pink on top of one blue). When we see Vanessa and her friend, Janet, fighting Theo to get through the door, the pink jacket obviously falls down. When Dr. Huxtable comes in, the jacket is seen hung up again, but when he leaves, it is on the floor again.
Twinkle, Twinkle Little Star - S4-E15
Continuity mistake: In the opening scene, Vanessa pours a full glass of milk. In a shot a few seconds later, she is pouring the same glass about 1/4 full.
Waterworks - S4-E18
Continuity mistake: Rudy blows bubbles in her milk and the bubbles rise above the glass. The shot changes to Vanessa and when it goes back to Rudy the bubbles are gone.
Continuity mistake: Cliff and Clair are talking in the living room while Cliff is making a sandwich. As Clair asks Cliff if he would trust her with a sexy man, Cliff starts putting potato chips on his sandwich from a bowl. As he takes the chips out of the bowl, the level of chips goes down. When Cliff asks Clair if she would trust him with a nineteen year old woman, the bowl is miraculously at the level it was at the beginning of the scene. But as Cliff finishes talking, the bowl is back to it previous level.
Chosen answer: That is the song "Open the Door Richard."