In Friends S05E19, Joey's grandma comes to visit. He was supposed to be in a TV series, but the director cut his scene. Joey brings a newly recorded tape for his grandma. We see Chandler singing David Bowie's Space Oddity, and Joey and Chandler rush to shut off the VCR. What did they not want Joey's grandma to see?
The truth they didn't want to show was that the scene on TV was faked. If grandma sees singing Chandler right after Joey's appearance, she would have understood that it was not a real broadcasting (because Chandler was not an actor and couldn't be in on TV in the same series).