19Prue's Permanent Disappearance
The beloved character of Prue Halliwell lost her life in the finale of the show's third season. She never again appeared on the show, in any form. If something like this happened in Grey's Anatomy, it would be understandable, but the Halliwell sisters communicate with the deceased on a
regular basis and often travel through time. You'd expect the remaining Halliwells to try to reach their departed sister at least once.
Fans of the series know that the creative decision to remove Prue from the remainder of the show was rooted in the fact that the actress who played Prue, Shannen Doherty, left the series was unwilling to return to the series under any circumstances.