Which comedies pulled a downer stunt that hurt the show?

Which comedies pulled a downer stunt that hurt the show?

I thought the series finale of Roseanne and the father’s death on Good Times killed the spirit of what had been strong sitcoms. Are there any comedies that you felt pulled a downer stunt that hurt the show? — Matt T.
One prime offender: the jarring Diff’rent Strokes episode in which Arnold’s best friend, Dudley, was molested by a pedophile. This kicked off a trend of ”very special” episodes (remember Alex’s amphetamine addiction on Family Ties? Or the death of downstairs neighbor Mr. Heckles on an odd, Hallmark-esque Friends?) that always slow down a show’s comic momentum.

(Got a TV-related question for Gillian? Post it here.)