Author Existence Failure: The original manga was never finished due to Ishinomori's death. Thus, the final planned story arc, "Conclusion: God's War" was never finished or published in manga format in the year 2000 as Ishinomori had hoped for (as his death cut the planning short). That is, until 2012, when Ishinomori's son Jou Onodera used his notes to complete it as a novel (with the anime writers having previously adapted it as a mini-arc and OVA). It has now been completely adapted into manga format and published on Shogakukan's free manga hosting website. Do keep in mind that this is all happening only in Japan.
However, Ishinomori's planning notes are also said to differ from both Onodera's novel and manga adaptations to various extents, leaving it iffy on how faithful the final story may be at delivering on his intent.
Hey, It's That Voice!: The dub of the 2001 series has many of these, even some in the exact same voice that some actors may have been known for in other anime dubs. Take any voice and look it up on Wikipedia and there will be a full list of roles that the voice actor has performed. Some will be more familiar than others.
Keep Circulating the Tapes: Only the first eight episodes of the entire 2001 anime were released on DVD in the US (the UK and Australia were a bit more fortunate and got the entire first half). Outside of bootlegs and internet uploads (which still don't cover the episodes after Episode 34, Episodes 38 & 46-48 not withstanding), the rest remain elusive. And only ten manga books have been translated into English, with Tokyopop's release only covering up to the end of the Underground Empire arc (which itself was going to be the original ending to the series).
The Other Darrin: Joshua Seth was unavailable to voice Joe in episodes 5 and 9 of the English dub for the 2001 series, as he was sick during the sessions. Derek Stephen Prince filled in for the role.
What Could Have Been: As mentioned in the Aborted Arc trope, "Angels" was originally going to be an arc to wrap up the manga, but it was unfortunately cut off on a cliffhanger when the magazine it was running in (Monthly Bouken-Oh) dropped Cyborg 009 as a feature. Ishinomori attempted to rewrite and revisit the storyline a few years later with "Battle with the Gods" (with it serialized in COM), but that was also cut short when the manga was dropped again.
Before "Angels", the Underground Empire arc would have ended the manga with the now-iconic and infamous "shooting star" sequence. Before that, fans believe the Mythos arc was meant to be an earlier wrap-up, as the arc ends very abruptly with an explosion and a narration stating that there were no traces of either Cyborg team left behind (leading to the assumption that the 00 cyborgs were all dead). This coincided with the manga being dropped from Weekly Shonen King, presumably leaving Ishinomori to wrap up the arc that suddenly.
Early draft designs for the characters had Jet/002 lacking the Gag Nose that he'd become known for, and Francoise/003 went through having several different hair style ideas before Ishinomori settled on her final look. Ivan/001 also lacked his Blinding Bangs, while Dr. Gilmore could have had an afro.