Deliberate mistake: During the final battle, at one point Sean kicks Crane off a rope bridge. When this happens, if you look closely you can see the rope railing on the side of the bridge fall away right before Crane falls. It's supposed to look like Crane is falling through the ropes, but they drop just a split-second too early (he's not even touching them when they start to fall), so it was done on purpose and mistimed. (It was likely timed to drop away just a split-second before Crane fell so that the stunt-person wouldn't accidentally get tangled in the ropes as they fell).
Guyver: Dark Hero (1994)
1 deliberate mistake - chronological order
Directed by: Steve Wang
Starring: Bruno Patrick, Christopher Michael, David Hayter, Kathy Christopherson
Revealing mistake: At the end of the final battle, when Sean rips the Guyver unit out of Crane's head and delivers his one-liner ("this model... has been recalled!"), you can see Sean's previously rigid, metal-like mask bending like rubber as his right shoulder rubs against it in the close-up. Particularly noticeable is the vent where steam sometimes shoots out, which bends nearly 45 degrees for about a half-second. (Revealing it's likely some sort of latex "stunt mask" for the actor to wear during fight scenes.)
Sean Barker: The Guyver Unit. It's been a year since the alien armor invaded my body and became a part of me. A year since I first became the Guyver and destroyed the Chronos Corporation and their shape changing killers, the Zoanoids. Now Chronos is gone. But the Guyver stays in me. Calling me to fight. To kill.
Trivia: Whereas traditionally many action-film series start out with R ratings and then later transition to PG-13 to broaden the appeal of sequels, this is one of the rare cases where a PG-13 original ("The Guyver") spawned a hard-R sequel made to appeal to the fanbase.