In the American version, during one scene Dr. Yamane's dialogue in Japanese contains the name "Godzilla" even though the monster hasn't appeared yet (revealing that this scene was originally later in the film).
Godzilla definitely has a height issue. Early in the film the Japanese scientist says that Godzilla is 400 feet tall. Later, Steve Martin states Godzilla is as tall as a 30 story building, which would put him closer to 300 feet. During Godzilla's 2nd attack on Tokyo, he is seen with 3 story buildings coming up to his knees, and 10 story buildings reaching his mid section, putting him at most at 200 feet.
As Steve Martin watches the "Godzilla" ceremony being performed on Odo Island, the same shots of the ceremony are repeated several times.
The helicopter that takes the first inquirers out to Odo Island is a different model from the toy one shown being destroyed during the subsequent storm.
During every ocean scene, using miniatures of ships or the monster, the exact same cloud formation is seen in the background.
When Steve Martin starts the tape recorder, he walks over to the window to describe Godzilla's attack on Tokyo. There is no cord connecting the microphone he's speaking into to the tape deck.
When showing pictures of dinosaurs they mention that 2 million years ago was the Jurassic Age. It was actually 200-145 million years ago.
The airplanes are seen on strings.
As Steve Martin and friend fly to Odo Island, the shot of them sitting in the helicopter was obviously filmed in front of a regular wall (the roof does not curve as it would in a helicopter, but instead goes straight up).
The photo shown at the press conference of Godzilla (which in reality is a painting) looks nothing like the real Godzilla, and is at an angle which the cameras on the hill could not have captured.
After Godzilla drops the building on Steve Martin's head, the tail can be seen through the hole in the wall with its puppet strings clearly visible.
When Emiko is talking to Steve Martin in the hospital, her blouse is one plaid when seen from the front (scenes of the original actress in the Japanese version), but a different one when seen from behind (an American stand-in added for the American version).
When Steve Martin calls his editor in New York and that editor is on screen, you can see that the wall behind him is the same kind of wall that is in the set of the Tokyo News Room that Steve is calling from (same set).
After watching the ceremony on Odo Island Steve and the security officer are laying in their tent. The wind begins to blow harder and harder until it is likely well over 30mph (50 kph). But when close ups of the two are shown inside the tent its walls are seen and they are not moving in the least.
After Emiko and Serizawa leave the lab, a different English-speaking actor's voice is heard dubbing actor Akihiko Hirata than the actor heard elsewhere in the movie.
During one scene where Godzilla breathes his radioactive fire, the nozzle that provides the spray can clearly be seen inside his mouth.
A monster that could potentially destroy the world would be the biggest news story of the century. And yet, except for Steve Martin (Raymond Burr) who happened to be visiting Tokyo, there isn't a single foreign news correspondent there.
The TV in Dr. Serizawa's laboratory goes from being off to playing with no one turning it on (the scene with the schoolchildren singing).
In his deposition upon returning from Odo Island, Dr. Yamani - supposedly Japan's foremost paleontologist - says that the Jurassic Age was "two million years ago," rather than the 144 million it actually was.
In one scene of the movie, in the court, where Dr. Yamane is giving some information based on his scientific expertise about the Godzilla, he pronounces the word "phenomenon" wrongly as "phenonemon" at least two times.