Revealing mistake: Near the beginning, Megalon steps out from behind a mountain. It's obvious the "mountain" is a cardboard cutout.
Godzilla vs. Megalon (1973)
1 mistake
Directed by: Jun Fukuda
Starring: Katsuhiko Sasaki, Hiroyuki Kawase, Robert Dunham, Yutaka Hayashi
Genres: Sci-fi
Trivia: The weapons used against Megalon (the ones that look like satellite dishes that shoot lightning) actually existed, although they were never used in combat. The Japanese experimented with them during WWII.