There was a line in a season three episode of "Ellen" where Audrey is late for work. Joe keeps getting after Ellen to give her "The Talk" (or something to that affect). When Ellen doesn't feel it's right, Joe mentions that she gave him the talk on his very first day. Ellen responds "It was supposed to be your second day. You were a day late." However, if you watch the 8th episode in season one, we actually see Joe's first day (it's also his first episode) and he's not a day late. But for the sake of a joke, they rewrote history.

Can you think of others?

On The Mary Tyler Moore Show, there's a similar thing. In two different episodes, Lou tells a story about why he hired Mary. Once, he says that she bumped into a desk and apologized to it. And later, he says she had a run in her stocking she kept trying to hide. But of course, we all saw when he hired her for real during the famous "job interview" scene. Neither of those things happened in it.

There's an episode of Friends from season 1 where Rachel remarks that she got a gay first impression of Chandler when they met at a birthday party of Pheobe's; which is odd, since they first met on the pilot episode, as Monica introduces her to everyone else at Central Perk.

Rachel: (To Chandler) "When I first met you... I thought maybe, possibly, you might be... but then you spent Phoebe's entire birthday party talking to my breasts, so then I figured maybe not."

On Reba, in season 1 it says she left Parker Renyolds to marry Brock. Then in season 2, it said she left Terry Holliway to marry Brock.
Also, in season 1, when Brock's father dies, he says how he left his mother when he was 12. But in season 3, Jake says grandma told the after he died she couldn't sleep well, because she missed the sound of him snoring. Meaning they still lived together.

There's an episode of Friends from season 1 where Rachel remarks that she got a gay first impression of Chandler when they met at a birthday party of Pheobe's; which is odd, since they first met on the pilot episode, as Monica introduces her to everyone else at Central Perk.

Rachel: (To Chandler) "When I first met you... I thought maybe, possibly, you might be... but then you spent Phoebe's entire birthday party talking to my breasts, so then I figured maybe not."
well, don't forget there was also the flashback episode in season 3 where they initially met at a sportsbar in the setting that would eventually become Central Perk about a year or two before season 1 started, and Rachel had the fantasy about Chandler

A lot of shows do this annoying thing. We think it's because new writers don't bother with looking up scripts from previous writers, even though they really should.

Family Matters: Steve was soon turned into the next door neighbor. But in his first ever episode, it was clear that Carl didn't know him yet at that point. Wouldn't he know the kid from next door, especially if it was someone as memorable as Steve? This happened because Steve wasn't supposed to be a regular at first, but he was so instantly popular, that they kept him on the show, and then they incredibly smoothly almost made us believe he always had been the kid from next door.

The Cosby show: What was the age difference between Cliff and Claire? Vanessa once said that he was four years older than her. But then, his 50th birthday was celebrated two years before her 46th birthday! Doesn't that make her six years younger than him? And from yet other episodes, it sounds like they were just the same age, because they were dating already as young teenagers.

Alot of I LOVE LUCY episodes were inconsistent in terms of keeping true to the previously established timeline. If you own the ILL discs, these "flubs" are captured in a special feature of the same name on each and every disc of the show's DVD sets.

On 'Mama's Family', sometimes Mama got the birth order of her kids mixed up. Here's the order:
*Mary Ellen
*Eunice
*Vint
But in one episode, it says Eunice is the youngest.

there's TONS of mistakes in "THE ODD COUPLE", some are: at the end of one episode, Oscar and Blanche are looking at their wedding pictures. Oscar says that he never returned the tuxedo he wore that day. But, according to a later episode, Oscar and Blanche got married when Felix and Oscar were in the army, and Oscar wore his uniform.
Another time, they mention that Felix stood in for Oscar in Oscars wedding pics., because Oscar was at a baseball game.
In the shows opening, the announcer says that Felix and Oscar are "childhood friends". But again, they met in the army according to one episode, but in another one they met when both served on a jury together.

In "THE DICK VAN DYKE SHOW" according to a 2-part episode, Rob and Laura got married when she was only 16 or something. But in all those flashback episodes, there's NO FREAKIN' WAY she's that age!
That show also has lots of other mistakes, ecspecially in the flashback episodes.

In the second episode of "NewsRadio" this scene happens:

Jimmy James: I love a good party. You love a good party, Joe?

Joe: That's why I went to college, sir. That's also why I didn't graduate.

Jimmy James: Me too.

Joe: That's also why I didn't graduate.

Jimmy James: Me neither.

Then in a season three episode, Joe tells an unconsious Jimmy about how neither of them graduated high school. Thus, history has been rewriten! I think there's also another mention of neither of them going to college.