In "A Fair to Remember", Lori gets jealous of Bobby and Lincoln hanging out because, according to Bobby, Lincoln's like the brother he never had. Leni suggests to Lori that the latter should hang out with Ronnie Anne, saying, "She could be like the sister you [Lori] never had." When phrased like that it sounds like a stupid idea. However, when phrased the other way, "You [Lori] could be like the sister [Ronnie Anne] never had", it actually sounds rather smart. Instead, Lori goes out with Clyde to get Bobby jealous.
"Vantastic Voyage" could have gone the Nothing Is the Same Anymore route by having the Louds buy a new and more efficient family van and stick with it. Instead, it has Lynn Sr. become so possessive of the car that, in the end, the family has no choice to bring back Vanzilla.
The entirety of "Schooled!" is considered this by many fans.
For starters, two of the three major storylines in the plot involve Lincoln and his friends starting middle school and Lori entering college, but there wasn't at least once scene or sequence (be it in this episode or a previous one) showing them graduating from elementary school and high school, respectively. Having had such a scene/sequence or an entire episode dedicated to such events could have driven the point of these storylines better.
A special mention goes to Lincoln going to school in Canada. Granted, it would've been a bit outlandish and out-there, which is probably why it was a one-time deal, but just the third quarter alone shows that that could've had loads of potential stories and jokes that revolved around it. Instead, he gets banned from the entire country after refusing maple syrup, putting the kibosh on that.
Many fans also have issue with how the episode handled Lori moving out. Prior to the events of this episode, it was built up to how she would have to move out of the house and say goodbye to her family. However, the episode not only rushes the goodbye scene to about 30 seconds, it makes the entire plotline of her having to get used to college into a rather minor subplot.
A more general example is Luna and Luan's relationship. While all the other sister roommate pairs have had whole episodes focused on how they deal living with each other, Luna and Luan's sisterly dynamic still remains rather unexplored. Arguably, one could interpret this as evidence that they have good enough chemistry to avoid potential drama, but it still sticks out since even Lisa and Lily are capable of having an episode dedicated to them ("Dream a Lily Dream") despite their relatively neutral relationship.
The entirety of "Cursed!", especially since the promos hyped it up as a crossover between the Louds and Casagrandes.
Except for Lincoln and Lori, the Louds hardly interact with the Casagrandes, and the latter family is even crammed into Lynn and Lucy's room later on in the episode. The Louds don't even contribute much to the episode, making the whole thing seem like a ratings grab.
Carl and Lola deserve a special mention. They're seen fighting each other, which seems to set up a chance for them to develop a friendship and understand each other. However, this plot thread never goes anywhere, making their interactions feel squandered.
"The Golden Curse" is a Wedding Episode and a half-hour special. But rather than using the opportunity to explore other branches of the Casagrande family tree or going into more depth with Mexican culture and family life, they instead spend their time on their relative's parrot's wedding while making Sergio, the most disliked character in the series, a major character in the episode.
