His father remarried?
At the time of his first roseanne apperance, johnny galecki (david) was also filming a pilot for another tv show in which his character was named david, roseanne producers originally wanted his character to be named david, but wanted to avoid confusion so named him kevin, but after his other series was cancelled they were able to go back to there original choice 
Did anyone else notice in friends that in the first episode when rachel leaves her wedding and comes to the coffee shop, monica asks her if she remembers ross, but just introduces her to chandler, but in season 4 or 5 a flashback shows that rachel knew chandler when she was in high school as he was friends with ross and she was friends with monica, and had spent at least 2 thanks givings with him at monicas parents house, and in season 9 or 10 it shows that rachel and chandler even made out at a college party, so if she already knew him, then why would monica have to introduce him to her
On Bonanza, producers were trying to give Adam a graceful exit (Pernell Roberts hated the show and wanted out of his contract.) They brought in a “cousin,” Will Cartwright, for five episodes and were all set to have him move permanently to the Ponderosa.
The Roberts decided to do another season of the show. Will Cartwright was never mentioned again. (The actor, Guy Williams, was offered what was supposed to be the starring role in Lost in Space.) At the end of that season, Roberts finally did leave the show, and Adam was never spoken of again.
A few years later, Dan Blocker died, and Hoss Cartwright was never spoken of again.
Life was cruel in Virginia City.
The best “in-universe” example is that Monica forgot Rachael and Chandler had met. Do you remember everyone who was at your parent’s Thanksgiving party a decade ago?
Not likely. Would I remember an in-her-prime Jennifer Aniston at my parent’s Thanksgiving party? Only a decade later? Undoubtedly!
I’ve been watching Daniel Boone on MeTV, starring Fess Parker. In the first two seasons of the show, the Boones had two children, Israel and Jemima. After the second season, Veronica Cartwright, who played Jemima, left the show and Jemima was never seen or mentioned again.
As I understand it, Louie’s inconsistencies are because he just doesn’t care. A lot was made of the fact that his ex-wife is black, but he wasn’t trying to make any points, he just picked an actress that he liked, and she happened to be black. Just because he had a pregnant sister in one episode doesn’t mean that he has a sister in any of the other episodes. Same for his brother.
Barney Fife had three different middle ‘names’ in different episodes: Milton, P., and Oliver.
DOOL has to be king of the Chuck Cunningham syndrome. Jed Allen played Don Craig from 1971 to 1985. A very major character for 14 years who told Marlena that he had to go drop a letter into the mailbox, and was never seen again and never mentioned in any dialogue on the show since.
I think both characters got one mention apiece after the actors’ departure.
Ben & the boys came home & found a man dressed in seaman’s garb inside with his back to them. Ben says “Adam?”, but it turns out to be a former shipmate.
The episode Forever was originally written to feature Dan Blocker. It became the first episode to air after Blocker’s death with Little Joe becoming the son who gets married & loses his wife. The character of “Hoss” is mentioned at least once, & Hoss’s photo is seen at one point. Bonnie Bedelia played the doomed wife.
Depending on how it’s used, not even that. The song itself is from 1934.
On the Andy Griffith show, there was an episode in which crooks who claimed to be interested in making a movie about Andy were actually casing the bank in preparation for robbing it. In a later episode, Hollywood producers came to Mayberry to make a movie about Andy for real. The producers were played by the same actors who played the crooks in the earlier episode.
Mommas, don’t let your babies marry Cartwrights…Ben’s good for at least three dead wives, Joe for one. And that’s not even counting the dead girlfriends!
Columbo’s never-seen but always-described wife can’t possibly look or talk like Kate Mulgrew, the star of Mrs. Columbo/Kate Columbo/Kate Loves a Mystery.
Law & Order: Jack McCoy has this really distinctive tie–thin, diagonal stripes of burgundy, navy and white. I only notice because I have one just like it. But it’s fun to notice which other characters (judges, defendants, defense counsel) wind up getting the very same tie from wardrobe when Sam Waterston isn’t wearing it. Since he wears bow ties on his new show, I guess this will become less of an issue.
I’ve always thought that one of the classics of this genre was The Odd Couple.
In the show’s intro for the first few seasons, the voice-over guy talks about Felix being kicked out by his wife and then “…with nowhere else to go he appeared at the home of his childhood friend Oscar Madison.”
A few years in, they did a flashback episode in which Felix and Oscar, both adults, meet while serving on jury duty. Subsequently the opening was changed to"…with nowhere else to go he appeared at the home of his friend Oscar Madison."
In one episode of MASH when Burns is trying to be tough, Margaret tells him “Oh, Frank! You look like my father just before he died!” Col. Alvin “Howitzer” Houlihan, USA, played by Andrew Duggan, turns up alive and well in a later episode.
In the original “Addams Family,” Fester was Morticia’s uncle. In the feature films, he became Gomez’s brother.
In the second STAR TREK pilot, the tombstone conjured up by Gary Mitchell clearly says “James R. Kirk,” not “James T. Kirk.”
In “Combat!” Sgt. Saunders was either from Cleveland, OH, or from somewhere in Illinois (though there is admittedly a town by that name in Illinois as well as in Ohio). In an authorized novel I once had (apparently based on the series’ writer’s guide), he was from Louisiana and his first name was “Orville” (“Chip,” the name he was called in the show, was evidently a nickname). He also gave conflicting serial numbers in a couple of episodes.
On MASH again, Gen. Steele and Col. Potter were two different characters played by the same actor (Harry Morgan).
She probably got married at 14 and moved over to the next hollow with her husband to start punching out babies until she died in childbirth. 
Possibly an Ashkenazi Jew (though I don’t think Klinger’s religion was ever mentioned).
They killed Kenny! You bastards!
(yeah, well, someone had to say it)