Nelson claims that no one went to his birthday party. This foreshadows "The Haw-Hawed Couple", where no one went to Nelson's birthday party except for Bart, who was forced to go by Marge.
The Radioactive Man TV series (the "campy '70s version") is an obvious reference to, and spoof of, the Batman TV series of the 1960s. Promotions prior to the blockbuster 1989 Tim Burton film Batman has said that it would "not be like the campy TV series of the 1960s".
The remark that kids would want to see the original Radioactive Man was a likely reference to Adam West, who had auditioned for the leading role in the 1989 film Batman, and had been taken into consideration on whether to stay loyal to the fan base of the 1960s show.
Comic Book Guy has a flying toasters screensaver on his computer during the first act.
He subscribes to the newsgroups alt.binaries.pictures.erotica, rec.arts.startrek.fandom, rec.org.mensa, rec.games.corewar, and the fictional alt.comics.radioactiveman.
Silly Sailor's Aquaworld lair is the set from Waterworld.
Gary has dark gray hair instead of dark brown in his cameo. Also, his glasses are a different design than usual, and lack the thick frames that they have in other episodes.
If Moe really was a part of the Little Rascals (who were actually referred to as "Our Gang"; "The Little Rascals" was the name given to the shorts when they were syndicated to television), then he would be really old (around 60-ish).
Bart was told he needed to grow an inch to get the part of Fallout Boy, but Milhouse eventually got the part, despite being shorter than Bart. One possibility is that for some reason the producers do not want Bart in the role, and make up the height excuse.
When the stunt of Milhouse being hit by the X-ray truck is filmed, the actual Milhouse appears to be riding the bike, with his distinct large nose and thick glasses. But when Bart runs up to "Milhouse" and finds out it's a stuntman, his face changes to the Estonian Dwarf, even when still wearing his Milhouse wig and (thinner) glasses.
When Milhouse is complaining about the repetitive nature of show business, for a few frames a piece of his hair is lighter blue like the mirror reflection as opposed to the bright blue that his hair is supposed to be.
During the closing scene in Hollywood, a teal-colored car disappears as it is driving.