That '70s Show/WMG

The inconsistencies in the timeline are because the show is the memories of the characters.
The gang from the show did a LOT of toking back in the 70s. It could very well be that what we see on the show are the memories of the characters in modern day trying to remember their experiences as teenagers in the 1970s--except that they fail in many respects due to their hazy memories.

Under this "hazy memories" umbrella:
 * T7S had eight Christmases from 1976 to 1979. The plot of each could have happened around the same time as each other, or some not at all.
 * The gang went skinny dipping and had their clothes stolen, and all rode home naked together in the Vista Cruiser; yet in a later episode Kelso makes a big deal about never having seen Donna topless.
 * Jackie's mother is played by Eve Plumb in a Season 1 episode, then is replaced with Brooke Shields in later episodes. This could be because the characters (particularly the guys) didn't notice her or her perceived hotness until she started dating Bob.
 * Ditto with Laurie, who was also Other Darrin-ed before departing.
 * There are various birthday and age inconsistencies.
 * The overall timeline of the 70s isn't adhered to with any major structure, aside from the final episode. It's all a blur to these guys!
 * Also, didn't Donna used to have a sister?

Season 8 was one big middle finger from the writing staff to the show's renewal.

 * Think about it: Season 7 was originally meant to be the end. It was planned out in such a way to end the series on a positive/bittersweet note wih Eric leaving for Africa. Everything was planned out... and then it was confirmed that the series was picked up for another season. The writing staff, after working on the show for 8 years, would be pretty pissed off that their "grand ending" was now a season finale, right? So, in retaliation, they decided to take this last season and structure it in such a way that pisses off enough fans to question WHY the show was renewed. ...
 * The start of darkness for the writing staff begins right at the end of the season 7 finale. Once told that the show is continuing, the writers rewrite the script and come up with an intentionally horrible hook for the following season: they break up Hyde and Jackie.
 * And then, learning that Ashton Kutcher forgot to renew his contract, came up with an annoying replacement for him and Eric, further twisting the knife: Randy
 * Or...

The whole show was just a purgatory for each character, where they tie up their unresolved conflicts in season 8.
The gang had been living pretty comfortably for 7 seasons, a status quo became established. Them BAM, season 8 shakes everything up. The cast face scenarios that entirely clash with what they identified themselves with, for 7 past seasons. Kelso finally has to finally grow up and accept responsibility, with his job and daughter etc. Hyde found himself slipping into a systematic routine of normality of having a wife. Eric had to face up to the realization that he couldn't hide away from life in his parent's basement forever, and begun experiences in Africa. Jackie found herself attracted to an ethnic minority, conflicting with her family's traditional conservative values. Donna found herself technically cheating, and thus displaying the same characteristics she would criticize a man for under Feminist doctrine. Fez I'm still struggling with, but I'm presuming Randy was an aspect of Fez' unresolved conflict that he was no longer the new stranger, and that Randy was achieving more than Fez did even as an obscure newbie. They all become changed people as a result of certain realizations, and when the clock hit midnight on new years' eve, and the 80s started, the gang all ascended to heaven. Season 8 might be a poor season from the fandom's perspective of what they're comfortable with, and certain aspects of it may have been painful to watch, but the purpose behind it could be the greatest thing That 70s Show has ever done.
 * That's probably the reason the timeline was cursed with inconsistencies (like 8 christmases in only 4 years). The 70s era was going to continue on forever, and ever, until the gang resolved their conflicts.
 * Maybe the first few seasons were real, then the watertower killed them, and they just sorta lived out the remaining seasons in their mind, when they were dead or in a coma. It explains how others seem to be killed by it, but they never do, because they're already dead.

Donna's younger sister, Tina had an active social life that kept her away from the house a lot.
And when Midge left, Tina went to live with her. Hey, it sure as hell beats her just vanishing from existance after 1 episode. As for Valerie (Donna's older sister who was in college and was mentioned once) she just never visited.

Donna got slut rabies.

 * Ok here me out. In the episode called "Cat Fight Club": Donna tried to break a catfight between Laurie and Jackie and got bit by Laurie. She makes a remark after that she hope she didn't get slut rabies. In that episode her hair was red. In the later seasons her hair became blonde. Coincidence or Brick Joke?
 * Coincidence; she dyed her hair blonde. ...although, considering how she acted during Season 8...

Sometime in the next couple of years, there's gonna be a show similar to this one about life in The Nineties.

 * Happy Days was a show made in The Seventies that was set in The Fifties. That '70s Show was a show made in The Nineties that was set in The Seventies. It only makes sense, doesn't it?