The "Cracks in Time", as seen in 'Doctor Who', are the over-riding arc to this first season of the Eleventh incarnation of the Doctor.
Here's some edited information about them from the TARDIS Wiki:
The cracks were described as "two parts of space and time that should never have touched". They were present in the very fabric of spacetime — a crack that appeared to be part of a wall would still be there if the wall were removed. (DW: The Eleventh Hour)
Some cracks released energy of pure time that could wipe individuals out of time itself and remove events from history, though time-travelers such as the Doctor still had the ability to remember them, at least so long as the removed event or person did not relate to the person's direct past. (DW: Flesh and Stone, Cold Blood) The Doctor guessed that such cracks had erased events such as the CyberKing walking over London in the Victorian era and the Dalek invasion of 2009, one of Earth's most publicly-visible invasions, explaining why Amy Pond did not know about those events.
Chronons, the elementary particles of time energy, were responsible for erasing objects from the present that no longer had a past. (DW: City of the Daleks) Whether this is related to the time energy seeping from a crack erasing individuals out of time is unclear.
You don't know what a boon this will prove to be for Toobworld Central! At last we have a unifying reason as to why certain TV characters not only vanished, but were not even remembered by the other characters left behind.
The best example of this is Chuck Cunningham, the oldest son of Howard and Marion Cunningham back in the 1950's. He disappeared from the show, and it could be suggested that he went off to college (although the show ran long enough for him to have finished school). However, in one episode, Howard gave thanks at the table before dinner,mentioning how fortunate he and his wife were in having two such splendid children, meaning Richie and Joanie.
No mention of Chuck.
Previously I theorized that maybe Chuck did something off-screen that was abhorred by his father that he was disowned totally; he would be worse than dead to Howard Cunningham - he wouldn't exist at all. (I also suggested that Chuck had been framed by his younger sisterJoanie, she of the "Crazy Eyes". And I pushed it even farther to say that she probably killed her brother and hid the body.)
But now? We can say that a crack in Time appeared back in the 1950's and consumed Chuck, removing from the memories of everybody in his immediate family - even for Marion, who gave birth to him!
This pop culture phenomenon has been named the Chuck Cunningham Syndrome in his honor. Wikipedia has a list of TV shows that have at least one example of this happening, although 'Davis Rules' and 'Grounded For Life' are missing. Many of these can be easily splained away when they're co-workers or neighbors or best friends. People get fired, find new jobs, move away, or just die. It's the missing family members that would best be served by the Chuck Cunningham Syndrome.
So I've gone through the list provided by that Wikipedia page and have chosen those cases of Chuck Cunningham Syndrome that could have been caused by the Cracks in Time.....
Sandra Clark (Jackee Harry) disappears halfway through the fifth season, no explantion of what happened to her or no mention of her name again.
All My Children:
Joe Martin's son Bobby went upstairs to wax his skis one day in 1970 and was never seen again. The show has been known to poke fun at the incident, such as one episode in which Opal ventured into the Martins' attic and found a skeleton with a shirt bearing the word "Bobby" next to a pair of skis.
Boy Meets World:
In early seasons, Topanga has an older sister, Nebula. When Topanga grows up and dates Corey, she is never mentioned again. When Topanga's parents get divorced, she is referred to as an only child.
Katherine Wentworth, Bobby Ewing's jealous ex-sister-in-law, stalks him at the end of the 1984-85 season and eventually runs him over with her car, killing them both. When Patrick Duffy returned at the end of the next season and the first episode of the 1986-87 season implausibly explained that the interim events had all been a nightmare of Pam Ewing, Katherine was never seen in the series, nor even mentioned, again - despite the fact that she, too, should still have been alive and stalking Bobby, one of several continuity problems created by this infamous plot line.
Days of Our Lives:
In 1985, Don Craig (Jed Allan, who had played the role since 1971) went out to mail a letter and never came back, and was never referred to again.
In 1991, Neil Curtis (Joseph Gallison, who had played the role since 1974) announced to colleagues that he had to see to some of his patients, and also disappeared without an explanation.
In early 2005, Cassie "DiMera" Brady (Alexis Thorpe) returned safely from Tony DiMera's European castle along with Jack Deveraux, Victor Kiriakis, and Caroline Brady. Soon after returning to Salem, Cassie disappeared with no reason. Her twin brother Rex left Salem after a fallout with Mimi, but Cassie's whereabouts remain unknown.
Leslie Carrington was beaten up by Sean Rowan and never seen or mentioned again
Everybody Loves Raymond:
Jennifer, Debra's care-free sister visits and says she will become a nun. She is never mentioned by Debra or her parents in any episode before or after.
Actually, this one could be splained away. She may have joined a cloistered order, which is why we never saw her again. And Debra and her parents might have talked about her whenever they were off-screen.
Judy Winslow (Jaimee Foxworth) disappears in the fourth season. She was last seen at Mama Winslow's wedding.
Bryton McClure, who played Richie Crawford, Rachael's son, appeared less and less once the character of 3J was introduced. By the last season, his character was completely dropped with no explanation.
At the end of Season 4, Florida Evans agrees to marry Carl Dixon and move with him to Arizona. They are unseen in Season 5, but when Florida returns at the beginning of Season 6 for Thelma's wedding, she is alone. Carl is never mentioned again, and Florida continues to be addressed as Mrs. Evans, not Mrs. Dixon.
Here's my splainin for this one: It turned out Carl Dixon was a bigamist. The marriage was annulled and Florida moved back home under her former name. Carl's serving time for bigamy and his name was verboten in the Evans household.
During the final season of the series (1970–1971), Ivan Dixon did not appear as the character Sgt. Kinchloe and the producers replaced his character without any explanation with Sgt. Richard Baker.
The King of Queens:
Carrie's sister moved into the house along with their father. The sister has a room upstairs and the father moves into the basement. At the start of season two the father is still in the basement but the room upstairs suddenly becomes Carries "office", and her sister isnever mentioned again.
The Toobworld splainin: At some point over that summer, unseen by the audience viewing at home, there must have been a big falling out with the sister.
Life With Bonnie:
The Molloys' oldest child, Samantha, disappeared from the show after the first season. According to series star Bonnie Hunt, this was due to "creative differences." The daughter seemed to have been replaced by a young neighbor whose parents are never seen.
During the first two seasons of the show, Mama's son Vinton had two teenage kids from his first marriage -- Sonja and Buzz. When the show returned for its third season, both kids were gone with only one fleeting reference made to them in the first episode. They are notmentioned at all for the remainder of the series. In the final season, while Vint and his second wife Naomi awaited the birth of their first child, it was implied that Vint had no other children.
Married... with Children:
The Bundys' youngest son Steven (Shane Sweet) runs away to live with the D'Arcys and is never seen again. His face is later seen on a milk carton.
My Three Sons:
A clear-cut example of Chuck Cunningham Syndrome that actually predated 'Happy Days' by a decade. From 1960-65, Mike Douglas (played by Tim Considine) was the oldest son of widower Steve Douglas. (The other two sons were Robbie and Chip.) When Considine left the series, Ernie Thompson (played by Barry Livingston) was adopted so that Steve still had three sons. While Ernie's adoption was referred to throughout the series, Mike was never mentioned again, and Steve Douglas mentioned several times that he had three sons, never four, and that Robbie was the oldest.
One Life To Live:
Carlotta Vega's adopted son Eli Traeger disappeared in 1998, never to be spoken of again.
During the first season, Maria had a younger brother who lived at home with Maria and her mother. He was never mentioned again after the end of the first season.
Step By Step:
The youngest Lambert boy on the show, Brendan, saw his role reduced during the last few years of the show (especially after the birth of Lilly) until he was completely dropped from the show during the last season without explanation.
During their move from Oklahoma to Seattle (and the show's name change to Almost Home), the family mysteriously lost two children, Steven Floyd and Ruth Ann.
The cracks in Time were said to have wiped out memories of major historical events as well - at least those events that only happened in Toobworld. So we can use them to splain away why nobody in the TV Universe ever seems to mention:
The Eugenics War from the original 'Star Trek' (although that could have been part of the televersion for the whole Balkan crisis of the 1990's.)
The original 1980's 'V' invasion
The Canamid invasion from 'The Twilight Zone' episode "To Serve Man"
The events of 'Space: 1999' did still happen - at least as far as the explosion of the fuel dump at the lunar colony which is a secret to the general public. But as far as the adventures that took place once the Moon left its orbit? That cataclysm was part of the coma dreams experienced by "survivor" Commander John Koenig.