In one of the Leave It To Beaver episodes, when Beaver is still little, Beaver has his friend Larry spend the weekend at his house. About halfway through the episode, Beaver and Larry get into a fight, and Larry wants to go home. The two boys are in the living room arguing while June and Ward are trying to figure things out.

When the two boys are in the middle of the screen with the parents on both sides, the boys are sort of in the back of the scene. Larry says his lines, then starts to say something else, but he quickly covers his mouth and acts a little embarrassed because it wasn't time for his part. Then when the camera cuts back to just the boys facing each other, Larry sticks his tongue out at Beaver. I think perhaps this is what Larry was about to do when he messed up. He jumped the gun, and it was very obvious if you are looking at the kids during that scene.