Continuity mistake: When Troy and Rowsdower are standing and looking up at the Lost City that flies up and leaves the planet it is dark. But a few seconds later, while they are still looking upwards towards the city, it is full daylight.
Continuity mistake: When Rowsdower is fighting the two cultists that were to execute him in the forest, he falls and gets grass stains all over his white jeans. When he meets Troy in the woods a few minutes later, his jeans are much cleaner, and the next day all of the stains are gone.
Other mistake: In the scenes that take place in populated areas, there are never any people seen anywhere in the streets. Not even during the "high-speed chase" (where the thugs' car does not quite manage to catch up with Troy on his ten-speed bike) in the town do you see anybody.
