- The general consensus is that "Fairly OddBaby" was the tipping point of the series. To a lesser degree, some feel the rot started sometime after "Channel Chasers", or as far back as Season 2 or 3 when Vicky made the switch from a bully of a babysitter to basically Satan in human form and for the infamous episode "Twistory" which was heavily criticized and subsequently banned from the airwaves for its negative depiction of British stereotypes.
- Seasons 4 and 5 used to be this while they were still running due in part to the Flanderization of some of the characters and the infamous episode "It's A Wishful Life", a twisted and very mean-spirited It's a Wonderful Plot where an unappreciated Timmy wishes he was never born and is shown a world where everyone's lives are better off without him. Now they are considered to be a part of the show's "golden age" by many fans. That said, "It's A Wishful Life" remains the most despised episode of the show to this day. Even the creator of the show, Butch Hartman, hates this episode.
- While most agree that Poof's introduction was the show's turning point, many believe Season 6's episodes aren't outwardly bad for the most part as it undid most of the Flanderization of Season 5. Then Season 7 came along, when the writers began forgetting about side characters and focusing only on Crocker and Timmy's Dad.
- Season 9 — the season where Sparky is introduced — has received a negative reaction. Besides Sparky himself, continuity errors started popping up more and more around this time ("Let Sleeper Dogs Lie" being one of the worst offenders).
- Season 10 was considered by many fans to be the lowest point of the series. Coincidentally, its also the last season before the show was cancelled. While many fans were delighted to learn that Sparky would not be returning to the show, it has still received its share of criticism. It doesn't help that the writing team for the season were new to the industry, or, in the case of Ray DeLaurentis, worked on legitimately bad cartoons.note It also doesn't help that there are long hiatuses between new episodes. Reasons include:
- The addition of yet another new main character named Chloe that Timmy must share his fairies with, who many view as an obnoxious Little Miss Perfect who makes Timmy even more of a loser and who many feel doesn't meet the requirements for being miserable enough to deserve fairies. Her "perfect" traits are thankfully toned down in subsequent episodes, causing some fans to warm up to her. That said, she still remains a divisive character.
- The Flanderization of several characters: Timmy is derailed into a selfish (and bratty) jerk once again to make his status as Chloe's Foil more obvious, Chloe goes from an excitable but relatively grounded girl to constantly hyperactive and prone to frequent bouts of hysterics and histronics, Cosmo and Wanda have taken several levels in dumbass and are constantly rendered helpless and unable to help Timmy and Chloe due to them losing their wands in every episode, Timmy's Dad has gone from a Bumbling Dad to a Lethally Stupid manchild who causes destruction wherever he goes, and Crocker's obsession with fairies (his defining trait since his debut appearance) is completely gone unless the plot specifically requires it, to the point that he no longer has a Freak Out! when he sees something magical nor does he say his famous Catchphrase anymore. The Season 8 episode "When L.O.S.E.R.S. Attack" has him being best friends with Foop despite the latter being an Anti-Fairy.
- On the subject of Timmy's Dad and Mr. Crocker, they make an appearance in practically every episode, even in ones not directly focused on them, leading many fans to grow sick of them.note In Mr. Crocker's case, it only got worse when the episode "Chip Off The Old Crock" introduced Kevin Crocker, Mr. Crocker's nephew, who is more or less a miniature version of himself.
- The overuse of pop-culture references, outdated memes, and 2010s slang in an attempt to pander to the tween audience.
- Poof's perplexing and inexplicable absence from the season. While Sparky being dropped from the show was confirmed by his voice actor, Poof is nowhere to be seen due to difficulties with having too many main characters in a plot. This is made all the more strange by the fact that Foop, Poof's Evil Counterpart, appeared in the season's very first episode and continues to make appearances throughout the season, even starring as the main villain in the episode "Blue Angel". Also, in both the old and new intros, Poof is still there. Poof finally appeared in the episode "Certifiable Super Sitter".
- The show's intro and theme song being changed to reflect the new season. Many feel that the new song is inferior to the original due to having just two people singing instead of a full chorus and lazily pasting together clips from episodes of the season. At first it seemed that the producers had caught on to the complaints and used the original intro in the episode "Girly Squirrely", but it only lasted for two episodes as from "The Fair Bears" onwards they began using the new intro once again.
- The sudden, mid-season shift from the series' traditional use of hand-drawn animation to Flash animation.
- The deterioration of the show's writing causing episode plots to become unoriginal and so generic and often being rehashes of plots from the show's previous seasons along with the previous season's continuity errors and problems.
Seasonal Rot / The Fairly OddParents!