Hayami Kouhei shows up in #13 in a dark red leather jacket - the color of Kamen Rider Garren. Even before that though, Fourze's henshin pose resembled Garren the most, and one Super Hero TaisenNet Movie has Kouhei getting the Garren Buckle!
In the net movies, Gentaro actually meets up with Garren and he and Tomoko see a video of Garren...who looks just like their principal. Garren insists that his name is Tachibana Sakuya, not Kouhei Hayami...which Gentaro thinks refers to Meteor's benefactor Tachibana, and his mind starts to overload from confusion.
Tokuda Yayoi, played by Rije from Abaranger, could count - Yayoi had issue with being forced through graduation, while Rijie suffered several forced age shifts.
Prop Recycling: Spares of the first four switches (Rocket, Launcher, Drill, Radar) sometimes get reused as pre-tuned switches - the MagPhone apparently started as a totally black Rocket and Radar. Then there's once-off examples like the gold switch that triggers the Rabbit Hutch locker and even the Core Switch.
Reality Subtext: As revealed by the production blog after the show wrapped, the scene at the start of the final episode where Yuki reads Kengo's farewell letter after he's been killed by Gamou, was the last scene filmed. According to director Koichi Sakamoto, the KRC's actors had become as close as family over the course of the shoot, thus the tears being shed were their real tears over the fact that they were about to go their separate ways. It was made worse was the fact that Ryuki Takahashi, Kengo's actor, couldn't be on the set for obvious plot reasons. The same blog post had Fumika Shimizu, Yuki's actress, reveal that she actually considered messing up her lines on purpose just so it wouldn't have to end.
Throw It In: Fumika Shimizu (who plays Yuuki) actually improvised the Hayabusa song, which the production crew allowed her to record (and used for a later episode).
What Could Have Been: Originally, Fourze was to have a much stronger anniversary aspect that what we were given. The original plan was to have past Riders appear and team up with Fourze. Sadly, this idea was scrapped because the producers thought it was too soon to have past Riders cameo after Decade.
Fourze breaks the tradition that W's FangJoker and OOO's TaJaDor of premiering the Mid-Season Upgrade in the Movie Wars film before showing it in the series proper. In this case Rocket States was shown in the movie but Fourze true upgrade is Magnet States, which premiered in the tv show proper.
Rocket States did later make a proper appearance in the series, in episode 41.