TV Shows that erased characters without explanation
1) Happy Days. "Chuck" Cunningham (oldest brother) did 2 seasons and then disappeared and not only that, was never referred to again.
2) Family Matters. The youngest daughter, Judy Winslow, did 4 years on this show and then they just didn't invite her back. With the show moving to Urkel as the star she really wasn't needed. What seems unfair is that they moved in her cousin Richie who was a cute little kid with lots of personality and he took her place. She was never mentioned again, almost as though she was murdered and it was a family secret.
3) Soaps. I've seen this a few times. Characters just don't work and they're gone or ignored in continuity like weddings. You'd expect someone's kids to turn up to their parents wedding but they're gone. Sometimes they're mentioned, sometimes they never existed.