Tsukasa was shown holding a Kamen Rider Kabuto Rider card after he defeated that Rider, even though he should already have had one from his encounter with Kabuto during the series. Indeed, he uses Clock Up, Kabuto's signature attack, to fight Super-1 and Kabuto. He also gains further cards for defeating Ryuki, Blade and Kuuga.
Despite watching Decade and Diend struggle to fight the Dai Shocker forces, Yuusuke makes no attempt to help them as Kuuga, leaving Natsumi to cut out and build the Decade Bazooka.
There would be a much more obvious answer to Narutaki's question about a Shark Rider; Kamen Rider Abyss from Ryuki's World.
Despite wanting to be a Rider here, Natsumi has already been Kamen Rider Den-O Gun Form, although Ryutaros was controlling her body. She gets her wish in Kamen Rider Decade The Movie: All Riders vs. Dai Shocker.
While teaching a man to transform into Den-O, the Den-O belt makes an error sound more commonly associated with the Faiz Gear.
It is not explained how Shotaro and Phillip/Kamen Rider W got to Dai Shocker's World from Futo City and back.
In the photograph of Sayo at the door to the house, Tsukasa's reflection in the glass does not show his true position; he was holding his camera in front of him, but the reflection showed him stood with his arms down.
Strangely, Deneb never refers to Yuuto by name, even though he usually says it frequently.
Everyone seems surprised to learn at the end of the movie that Yu is actually the younger version of Yuuto, yet Deneb would not have been able to possess him had he not been the same person he had/would make a contract with. Presumably Yuuto's disapperance severed their link, allowing Deneb to form a new contract with Yu.
As the battle restarts after Sieg and Tsukasa arrive, Den-O Wing Form stumbles on the rocky terrain and is helped along by one of the enemy Gelnewts.
It seems strange that Owner would issue DenLiner tickets that allow admission to the train but not to travel on it.
Presumably Yuuto has always known about Deneb, DenLiner and it's passengers from his encounter with them as a child (as Yu), but did not say anything so as not to influence future events. This must also mean that he has always known that those he met would survive whatever happened up to that point, such as the many attacks from Kai and the Imagin, Gaoh's hijacking of DenLiner and Hana and Ryoutarou's regression into children.
Ryoutarou tells Yu that using a DenLiner ticket will take him to "DenLiner's station". Yet when he does use it, he ends up effectively in the middle of nowhere in the sands of time with DenLiner parked nearby with no station or any other structures in sight. This is however, exactly where Ryoutarou found himself when he first accidentally used a ticket at the start of Kamen Rider Den-O.
It is unclear where all the laser blasts and explosions were coming from during the final battle as all the battling Riders were visible and not shooting anything. It is possible that entities such as Dragreder, Castle Doran or even DenLiner were responsible even though they were not shown as Natsumi has previously seen them fighting in the battle in her dream.
Natsumi's dream of the Rider War is different from the reality as it does not include Blade King Form, Kuuga Ultimate Form or Diend's involvement. Also, no mecha appear to be present during the actual battle.
During the Rider's battle with Apollo Geist, Decade appears to refer to Kamen Rider Kiva as "Asumu" and Kamen Rider Hibiki as "Wataru". It should be the other way around.
Despite appearing on the picture portal, neither Dragreder, DenLiner or Castle Doran appear during the final battle with Decade.
Strangely, none of the deceased Riders from Kiva or Blade's World are ever referred to by their real names.
For the first and only time, Decade Complete Form and Ryuki Survive Form do not move in tandem as they attack the Thorn Fangire.
During Apollo Geist and Yuki's wedding, twenty people are shown watching the service, but when Geist attacks them for their Life Energy, only 16 such energies are shown entering his body.