These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
A: Fatty Arbuckle makes an appearance in the audience, and Johnny makes a comment that he will have a long, good career. However, if you know anything about Arbuckle, you'd know that in 1921 he was arrested for allegedly raping and killing a young actress at a party. Despite the fact he was innocent, the controversy effectively (among other things) ruined his career. This fight took place in 1928, long after his career had been destroyed.
B: Johnny makes a comment about how the train Keaton uses to win is from "[his] masterpiece, The General". The General, despite being considered a classic now, bombed when it came out in 1926'', just two years before this fight.
One episode had Benito Mussolini appearing and for some reason wearing a pickelhaube. Why an Italian would be wearing a German helmet from the First World War is not addressed.
Ensemble Darkhorse: Stacy Cornbred, especially after her death, and even more so after the fan reactions to Debbie and Tally's Jerkass behaviour. It was most likely the reason behind her coming back from the dead in a Season 3 Halloween episode (and the video game.) As noted in Seasonal Rot below, many fans felt that her death was the point where the show jumped the shark.
Epileptic Trees: It was strongly implied that The Masked Man was Michael Jackson, as his pale skin, high-pitched voice, eccentric dance moves, and habit of seductively leering at little boys (often distracting him long enough to allow the other guy to get a hit in, causing him to flee screaming) were all dead giveaways. Despite this, nobody could ever quite figure out who he was.
In the Penn & Teller vs. Siegfried and Roy fight, a tiger is used as a weapon at one point. Since Roy's real-life mishap with the tiger that brought their show to an end, parodies in which their tiger goes berserk just don't seem quite as funny, even if everyone knew it would one day be a reality. (That includes the one on the Simpsons episode where Springfield legalizes gambling and Mr. Burns opens his own casino.)
During their fight, Medusa tail-whips Steve Irwin in the chest and sends him out of the ring. Once he recovers, he reaches inside the wound, holds his heart out, and then puts it back in its place, saying "What a stinger!" Yeah, not so funny now that Irwin's death was caused by a stingray inflicting a fatal chest wound (and, not, as othershows have predicted, by being eaten by a crocodile).
The Game getting shot by 50 Cent became less funny when it was revealed that The Game's brother actually had been shot shortly before the fight. The Game's father even criticized MTV for airing that episode.
Hate Sink: Tally Wong seems to be this, due to her Jerkass behaviour.
The whole premise of that fight counts. At the time, Titanic had just come out and everyone expected Leo to be the next flash-in-the-pan pretty boy. Yeah, yousee, aboutthat...
In the Britney Spears vs. Christina Aguilera match, Britney rips Christina's hair out, leaving her bald. Kinda ironic, considering that Spears would be the one who ended up bald when she went on her infamous meltdown between 2006 and 2007.
A decade or so ago, during the Time Travel episode, there was a fight between Sherlock Holmes and Jack the Ripper. Today, there is this...
During the fight between David Arquette and Paul Reiser, David's sister Rosanna calls him, demanding to be in Scream 4. David's response? "Rosanna, I told you: 'No more Scream movies!'" And in comes 2011...
Seasonal Rot: Though some feel that the show jumped the shark when Stacy Cornbred was killed off, the later episodes of the original run were still considered far superior to the revival that aired on MTV2's Sick-a-Mation lineupnote Which also included other short-lived shows like Where My Dogs At and Pedro and Flocko, and reruns of Beavis And Butthead and 3 South., which was so bad it's considered Fanon Discontinuity. The reason? Eric Fogel wasn't executive producer.