When the previously cute child cast members of a television sitcom begin to go through puberty, thus robbing them of many of the traits that made them appealing in the first place.
The definitive example is when Jerry Mathers on the 1950s sitcom "Leave it to Beaver" went from adorable child to awkward, gangly teenager over the course of several seasons.
On the current (fifth) season of "Modern Family" the actor who plays Luke is going through puberty. His voice is changing and he's gotten to be really odd looking. Meanwhile, the breasts of the actress playing his middle sister have grown at an alarming rate. It's gotten difficult to buy her as a high school student. They used to both be such cute kids. The show is clearly suffering from "Leave it to Beaver Syndrome."