Plot holes: The Kilaaks exist in a temperature of at least 7,000 degrees. But several humans captured by the Kilaaks from Monsterland are in both Kilaak bases on Earth and the Moon. They should have been incinerated.
Miscellaneous: When the reporter announces that Baragon is attacking Paris, it is actually Gorosaurus. The Baragon suit was loaned out to Tsubaraya Productions for use on one of their television series and wasn't returned in time for filming.
Revealing mistakes: When Godzilla attacks Tokyo and is being shelled, a building is hit and its side blown off. Windows warp, but remain in place, revealing they were plastic.