Reader Fred Nolan points us to Wikipedia's entry on the Very Special Episode. Some highlights:
Family Matters - When Laura gets assaulted outside of school, a guy at school offers Laura a gun, Urkel tries to stop her. This is one of the most serious episodes of the series, the only comedic scene featuring Urkel rapping. It even ends with a message from Jaleel White about guns.
Facts of Life - Natalie is almost raped by a clown on Halloween.
Perfect Strangers - Balki is sent to an internment camp after his island nation of Mypos declares war on the United States.
Television viewers and critics alike widely regard the term very special episode as ironic and melodramatic.