Acting for Two: What happens briefly when Diend whisks the Black incarnation of Kotaro to the world of Black RX, next to that world's Kotaro.
Not to mention the brief period in Kabuto's world where Masahiro Inoue plays both Tsukasa and the Worm that's copied his form.
This was apparently supposed to happen again in the original draft for The Last Story, which would have featured an Evil Twin of Tsukasa, but obviously it was removed in the rewrite.
Amusingly, in the "Henshin!" segments of the All Riders vs. Daishocker net movies, Narutaki's actor plays the man-on-the-street whom Tsukasa teaches to transform. Yes, that's him in the loincloth screaming "AAAAAAMAAAAAAZOOOOOON!!"
The roses in Ibuki's transformation may be a reference to the fact that Jouji Shibue's first big role was Tuxedo Kamen in Pretty Guardian Sailor Moon.
Beam Me Up, Scotty!: The now-legendary ONORE, DICKEIDO! was only said in the actual show once, with Narutaki generally saying it in the Net Movies. However, it's now considered Narutaki's catchphrase by the fans.
Enforced Method Acting: Might be something different but both Ryuga and the Original Blade have trouble sliding the cards into their belt buckles.
To be fair, you can notice the same sort of problems if you go back and watch Kamen Rider Blade.
This actually gets a nod in the shorts made for All Riders vs Daishocker, where Tsukasa is teaching a man how to transform into Ryuki; he has trouble slotting the Advent Deck into the V-Buckle, so Tsukasa tells him "Don't worry, let the cut take care of it." This is followed by another attempt, with a Smash Cut interrupting the motion and showing the deck being slid perfectly into place.
Executive Meddling: Movie Wars 2010 was initially announced as a double feature with a Decade movie called Decade vs All Riders and a Kamen Rider W movie called Episode Zero. Eventually, the entire project was reformulated, turning it into a triple feature and heavily changing the overall content of the Decade story, which lost the All Riders label and ended up very different from the original preview, even having to basically ignore the some twists of the show's cliffhanger ending due to the shorter running time.
Put it this way: after the final episode of the TV series ended on a Cliffhanger, they immediately showed a preview for Movie Wars 2010, billing it as "the final episode of Decade's story". Not one scene in that preview was in the final movie. They did include the scenes in the director's cut, but it was just the same scenes from the preview.
Several parts of Decade's story, most notably the presence of Electro-Wave Human Tackle, only were kept because they had already made props for them before the change of direction of the production. Tackle was initially supposed to search for Stronger after he disappeared fighting Decade, and that story even got published in some of the earlier previews, but her storyline was completely changed after it was decided that Decade's movie wouldn't feature All Riders anymore, even if a new Tackle costume was already made
There's also Takayuki Tsubaki (Kazuma Kenzaki), who was contracted for the movie, but ended up just doing a few generic yells as Blade in a very quick scene, meaning most people didn't realize that was the original Blade fighting Decade.
Milestone Celebration: The tenth anniversary of the Heisi Riders was celebrated in the biggest way possible. And with the Showa Riders getting involved in the crisis by episode 26, this celebration was taken to the next level.
As we noted above in this page, Tsukasa is a Cloudcuckoolander. Applicable to both Kadoya and Hiiragi.
Unlike other characters who were altered in the new reinterpretations of the Nine Worlds, there is a Reiko Momoi in the Ryuki World. And there also was a Reiko Momoi in Kamen Rider Ryuki. While Decade!Reiko was Da Chiefand Stuffed in the Fridge to kickstart the plot, Ryuki!Reiko was the Intrepid Reporterand the worst thing to hit her was pinkeye.
Takumi Ogami shares a family name with GaoSilver, who also had a "wolf" theme to his character.
Promoted Fanboy: Masahiro Inoue, the actor for Decade, is a huge fan of Kamen Rider, even as an adult. When the Black/RX episodes aired, he made one post on his blog consisting of little more than excited babble about Apollo Geist and "I'm Joe the Haze!". Adding to this, when the finale aired, he expressed relief that he can "enjoy Kamen Rider objectively again". Not to mention his buying a swimsuit with the Decade colors, or various other eccentricities... (Rider Kick on a trampoline?)
On the same note, Ryota Murai, Yusuke Onodera/Kamen Rider Kuuga, was a huge Kuuga fan as a child. You can imagine his delight at getting to be Kuuga himself as an adult. Combine this with Inoue's fanboying, and you get some very interesting blogposts.
When Super Hero Wars: Kamen Rider vs. Super Sentai was announced, Inoue made a blog post where he talks about how he missed being a Rider and how much he looks forward to getting back to action.
You can catch the motorcycle used as Machine Decader in Kamen Rider Kiva., ep. 32 and 35 (used by Nobori Taiga/Saga.)
Reality Subtext: Try watching the ending with the knowledge that Joe Odagiri, the actor of the original Yusuke/Kuuga, was the only Kamen Rider lead actor to have explicitly expressed an Old Shame towards the franchise.
What Could Have Been: The producers said they wanted to make a modernized Riderman costume for Gackt to wear in All Riders, a la the suits for Riders 1, 2, and V3 seen in Kamen Rider: The First and Kamen Rider: The Next, but weren't able to complete it in time for filming, which is why he appears only as an untransformed Joji Yuki while Riderman is portrayed by a stuntman during the ending carnage. However, they were able to create a modernized costume for Tackle.
During the show's run, rumors were constantly flying over what the AR versions of familiar Rider worlds would be; probably the best one claimed that the World of Kabuto would have Tendou and Kagami's counterparts as competing television personalities, Tendou hosting a cooking show and Kagami a variety show.