Inferred Holocaust: Kuuga's Rising Mighty Kick causes a gigantic explosion, which at one point happens in a heavily urban area. This is the point in which the show begins to show that Kuuga becoming even more powerful isn't a good idea... While no one is actually killed in the blast due to Godai informing Ichijo ahead of time that his kick might be too powerful, the property damage leads the police to begin doubting No.4's status as an ally.
In Memoriam: Before the Heisei Rider era began with Kuuga's premiere, a message says "This project is dedicated to Ishinomori-sensei". Shotaro Ishinomori was the creator of the Kamen Rider series, and Kuuga was an idea he had to revive before his death.
What Could Have Been: There was supposed to be a theatrical film set after the events of the TV series, but the project languished in development hell when the writers couldn't come up with a satisfactory way to continue the story and the production was eventually canceled in 2006.