1. Red catches the gang smoking pot... twice. 

In the season 7 finale, Red comes down to the basement to find the whole gang smoking pot, as usual, and his rage is even carried over into the season 8 premiere when he goes to find all their stashes and Kitty, rather than flushing them, decides to experiment. This would all be absolutely hilarious, if not for the fact that Red already caught them smoking in season two. It was the entire plot of two episodes in season 2 and 3!! His being so shocked and appalled to find them doing it again in season 7 makes no sense!

2. Eric gave Donna too much space... except he didn't! 

In season 5, when Hyde isn't sure how to approach Jackie while she's distressed about her father, Eric and Donna offer advice based on how they dealt with each other after Donna's mother left in the previous season. It turns into a hilarious argument about how Donna needed Eric, but he was giving her "space", assuming it was what she wanted. He told her that if she needed him, she should've gone to him, and she argues that she couldn't find him. Hilarious, right? EXCEPT THEY WEREN'T EVEN TOGETHER WHEN HER MOTHER LEFT!! They had just broken up, and on top of that, Eric most certainly did not give Donna space. He came over to her house to drop off a casserole and they ended up having sex, twice. Later, when he went to confront her about it, they had a friendly, heartwarming moment where he helped her fold up her mother's clothes to try and be there for her. This was THE PREVIOUS SEASON, not like 4 years earlier. Come on, writers!!

3. What year is it?? 

The first few seasons don't do the worst job at keeping track of what year it is, though holiday specials do tend to throw the whole thing off balance. But it was 1979 for like three whole seasons, and it felt like the writers were just dragging that year on as long as possible to keep the show going. A whole year goes by between Christmas and New Year's Eve! Honestly, the way most shows handle "what year is it" frustrates me (like, y'all started high school in 2008 but didn't graduate until 2015??), but this show wasn't set in present day, so they could have had a much easier time writing the plots into a consistent timeline.

4. Just... all of season 8.

This one isn't so much an inconsistency as it is a LOT of sloppy writing. Randy's character exists solely to be a love interest for Donna, and not even a good or well-written one at that. While I'm not anti-Fez and Jackie, her relationship with Hyde was given a sloppily-written ending, and his storyline with Sam made absolutely no sense. The only good episodes were "Stone Cold Crazy" and the finale. I will die on that hill.