Revealing mistake: During the nighttime forest fight-scene, when the Zoanoid rips the tree out of the ground, it's obviously a lightweight prop. Furthermore, when he starts trying to swing it at the Guyver, both ends of the tree are broken off cleanly... which doesn't make sense since the top half should be covered in branches and whatnot.
Revealing mistake: When the security guard is killed near the end of the opening scene, watch when he gets shot... you can see small flashes/sparks going off on his chest from the squibs at least twice.
Revealing mistake: During the final battle, at one point Crane picks Sean up by the neck and kicks him in the side, sending him flying onto a nearby rope bridge. When the camera cuts and you see Sean hitting the bridge, watch closely... he's coming from a completely different angle than the one Crane kicked him in. In the first shot, Sean is kicked diagonally towards the bridge from about a 45 degree angle. In the next shot, it's clear the stuntman was standing at the edge of the bridge and just jumped forwards to land. It goes by quick so you might not notice it the first time, but it's there.
Revealing mistake: In the final battle, at one point Sean gets kicked through the boards on some scaffolding, which in turn makes the whole thing fall over dramatically in slow-motion. When Sean flies through the boards that are holding the scaffolding up, however, they way they break and react reveals that they are cheap, light-weight breakaway boards. They break like nothing, don't splinter at all and before the shot cuts, you can even see them wobbling and shaking like they're made of styrophone.