Austin St. John's real name is Jason Geiger. He used the stage name Austin St. John to alleviate confusion on set, as there was already two actors and his own character, all with the same first name.
Dawson Casting: Do you honestly know any high school freshmen that look that old? Well, out of the original five, Austin St John was 18 and Thuy Trang was 19, but Walter Emanuel Jones and Amy Jo Johnson were 22 and David Yost was 24. If they had made them high school seniors like they originally were, it would have been more believable.
Made worse by the fact that the characters were retroactively de-aged from seniors to freshmen after the first season was a success. Originally Austin St. John and Thuy Trang (and Jason David Frank (19 at the time)) were going to be plausibly the same age as their character.
The second season wasn't as bad, but still fairly notable. Of the four (including Kat) replacements, Johnny Yong Bosch was 18, Karan Ashley was 19, Steve Cardenas was 20, and Catherine Sutherland was also 20. Add in the remaining 2, and you get another 20-year-old (Jason) and a 25-year-old (David). Before Kat, however, Amy was 23 and still on the show.
Robert Axelrod voiced Finster and Lord Zedd on Power Rangers, as well as Armadillomon and Wizardmon in Digimon
Lost Episode: The original Pilot for Mighty Morphin' Power Rangers was advertised and presented as such on Fox Kids. Notable changes include the casting of the Yellow Ranger as Audri DuBois as opposed to Thuy Trang, Zordon being known as "Zoltar", and the Zords being referred to as "Droids".
Mean Character, Nice Actor: Bulk and Skull are two terrible bullies, but their actors (Paul Schrier and Jason Narvy, respectively) are said to be some of the nicest and most fun guys you could ever hope to meet. One PSA even did An Aesop rather effectively about this. Of worthy note is that as the show goes on, Bulk and Skull go from being annoying jerks to comic relief, and as time passes even further, they go from okay guys to actual heroes.
In a bit of a subversion, there's Tommy Oliver. While a nice guy and not a villain proper, he is quite famous for his dark beginnings, where he was brainwashed into being mean, rude, and very cold. The actor, Jason David Frank, is just as nice of a guy (if not moreso) as the character he (normally) plays. It's worth noting that in later seasons, his character strikes fear into Bulk and Skull, and becomes more and more of a gruff stoic. Meanwhile, his actor is up at panels cracking jokes and relating funny stories to his fans.
The Other Darrin: Rita and Scorpina became unusable for awhile after Season 1, once all the Zyuranger footage with Machiko Soga and Ami Kawai ran out. Rita would became a regular character again after Carla Perez was cast for the role with Barbara Goodson still providing her voice, whereas Scorpina only returned once since the producers were unable to get a hold of her replacement actress (Sabrina Lu) beyond "Goldar's Vice-Versa".
The Other Marty: Audri DuBois was originally cast as Trini in the original pilot. Also Skull was originally played by an actor other then Jason Narvy; while he was not named in the pilot, his biker attire and crush in Kimberly identify him as the same character. However, Bulk and Skull's personalities were reversed in the pilot.
The Pete Best: Brad Hawkins was originally set to play the White Ranger, who was originally intended to be a new character separate from Tommy the Green Ranger. Due to Tommy's immense popularity on Power Rangers, Tommy was instead written back on the show as the White Ranger, while Hawkins was given the lead role on VR Troopers, which was originally intended to be a vehicle for Jason David Frank.
Postscript Season: Season 1 after "Doomsday". It was supposed to end with a shot of the Rangers sealing Rita away again, and is why Tommy makes a cameo appearance at the end. The rest of the season was new footage created by Toei specifically for Power Rangers.
Prop Recycling: Old monster suits were often reused and altered in some fashion to create new monsters.
Mr. Ticklesneezer was painted blue and dressed in a Santa Claus-like outfit to become Grumble the Elf (who was more of a friendly elf creature rather than an evil monster - but then, so was Mr. Ticklesneezer).
Chunky Chicken had his feathers plucked and had armor and a helmet added to become Turkey Jerk.
Grumble Bee had his red sections painted silver and was given buck teeth to become Waspicable (a peaceful monster who didn't want to harm anybody) in Power Rangers In Space.
Pirantishead, RoboGoat and Turbanshell were repainted and had a bunch of crap added to them to become random alien monsters in an episode of Power Rangers Turbo.
Primator was painted whitish and had extra fur added to him to become the yeti-like Snow Monster.
Salaguana was chopped into little bits and used as reptilian armor for Triter, a heroic alien warrior from Power Rangers Zeo.
Silver Horns was painted green and oddly had a tongue added, since he didn't really have a mouth, to become Repellator.
The Wizard of Deception's rat monsters were actually leftover, unused suits from the movie that were replaced by the Oozemen.
Inciserator was made up from the head of an unused Kakuranger monster and the body from Merrick the Barbaric (whose own episode hadn't even aired yet).
Tube Monster was Pipebrain repainted white and blue, but that is actually from Gosei Sentai Dairanger to show the cycle starting all over again with an almost identical type of monster.
What Could Have Been: At some point before Season 2, there were plans to introduce new versions of the Dinozords, as well as various different takes on the White Tigerzord (as can be seen by these pieces of concept art from a Super Sentai Giant Robot artbook). These plans were aborted when it turned out that adapting mecha footage from another Sentai proved to be more economical for Saban than commissioning Toei to make more new footage for them
A redesigned version of Titanus the Carrier Zord was also planned to be introduced for Season 3. Various concept arts◊ were drawn, but Titanus was left mostly unchanged in Season 3 (aside for the addition of the Dragonzord's chest plate, since the Dragonzord was absent during Season 3). Some of those designs were made for Power Rangers In Space, and indeed a "Space Titanus" toy was in fact released, though it was never used in the show.
A Samuraizord, shown in the same image above with the Titanus redesigns, was considered for the second season as well. The Samuraizord would've been very similar to the Red Dragon Thunderzord, in which it could transform between a Tyrannosaurus and a humanoid robot. Another reject concept art shows a robot that was formed from three bikes.
Dan Southworth (Eric Myers on Power Rangers Time Force) stated at Power Morphicon that he had auditioned for the role of Adam back during production of the second season.
Deleted Scene: Actors at Conventions have commented on various scenes filmed that didn't make it into the movie, such as the Rangers doing some preliminary training between being given the ninja robes by Dulcea and traversing into the forest. In particular, Tommy's was apparently seen practicing some superhuman kicks similar to the corkscrew kick he used to finish off the last gargoyle.
What Could Have Been: The producers originally intended for the Rangers' masks to not have their visors and mouth pieces, feeling that it removed any sense of personality. They realized that their changes were counter-intuitive to the point of wearing a helmet because they were supposed to be an elite, faceless fighting force. They went overbudget and over schedule to refilm the necessary scenes. This actually resulted in the second season of the show overlapping with the film production (they were supposed to finish filming it when they returned home to America), the last few episodes were filmed in Australia.
Austin St. John (Jason), Walter Jones (Zack), and Thuy Trang (Trini) were originally slated to appear in the film, but due to their contract dispute halfway through Season 2, they were dropped.
After Ivan blew up his Tengu Warriors for failing, he created a colorful-feathered "Queen Tengu" out of their feathers.
Catherine Sutherland (Katherine Hillard, the second Pink Ranger) auditioned to play Dulcea. Evidently they liked her enough to bring her to America for a role. Sutherland has lived in America since.
Mariska Hargitay was cast to play Dulcea after the original actress, Gabrielle Fitzpatrick, had to back out due to a scheduled surgery. The producers didn't feel that Hargitay was working out and was replaced with Fitzpatrick after she recovered. Due to budget issues, she wasn't able to refilm an extended sequence where Dulcea trained the Rangers in their ninja outfits at her home. As such in the movie the Rangers seem to just sit and stare at the monument for a time before deciding to go.
Ooze originally created a team of rat-men to battle the Rangers at the construction yard. The director and producers felt they looked horrible and very low-budget (their goal was to create a high-budget looking movie) and such convinced 20thCenturyFox to replace them with the more creepy looking Ooze-men. The rat-men costumes were later used in the "Return of the Green Ranger" three-parter on the TV series.