Sixteen months before the movie's release, on June 28, 1965, the Pan Am Boeing 707 (registration: N761PA) that Jeffrey Hunter is seen leaving from during the opening credits had an engine explode just after taking off from San Francisco. The uncontained engine explosion caused a fire, a fuel tank explosion and one wing partially separated. The plane made an emergency landing at Travis Air Force Base and there were no injuries.
Power asks Big Buddha if he has imported U-235, and Buddha proudly replies that he has U-238. However, Uranium isotope 238 does not generate an explosive chain reaction. Isotope 235 does that, and a considerable part of the Manhattan Project was employed to get enough U-235 to make a bomb. If Buddha wanted to blow up Los Angeles, the U-238 would be useless to him.
Reportedly Jeffrey Hunter gave up the Star Trek (1966) series to make this movie, in part because his wife didn't want him to do television and to focus on movies.
Jon Lormer, who plays the elderly professor, appeared with Jeffrey Hunter in the original Star Trek (1966) TV pilot, The Cage (1966), which most of it was shown in the two-part episode, The Menagerie (1966). He would then later appear in two other episodes, Return Of The Archons (1967) and For the World is Hollow And I Have Touched the Sky (1968). Robert Phillips, who plays George, also appeared in The Cage, and his small scenes were included in The Menagerie. In addition, France Nuyen, who plays Kitty, also guest starred on Star Trek where she played the title role in Elaan Of Troyius (1968). Maggie Thrett, who plays Second Sister, did Star Trek as well. She appeared in Mudd's Women (1966). Robert Ito, who plays Sato, would later appear on Star Trek: The Next Generation in the episode, Coming of Age (1988).