Corrected entry: I saw this bizzare incident while watching the movie on video. In the scene where the expedition reaches the shore of Mothra's island to plead for her help, numerous skeletons of creatures are seen in the background. As the group is talking about the effects of atomic testing on the island, the head of the reptillian-shaped skeleton in the center of the background begins to wobble back and forth unnaturally, possibly due to it not being properly mounted on the rest of the body.
Godzilla vs. Mothra (1964)
1 corrected entry
Directed by: Ishirô Honda
Starring: Hiroshi Koizumi, Akira Takarada, Yu Fujiki, Yuriko Hoshi
Correction: Actually, the "skeleton" you speak of is a malnourished giant turtle. It is moving its head.