The character (played by Jacques Dynam) of the main lieutenant of Comissaire Juve changes name between "Fantômas" (1964) - where he's called Michel, and "Fantômas se déchaîne" (1965) - where he's going by the first name Bertrand.
In the end, Juve is hanging from the plane's door with both his hands visibly empty in a wide shot. Then, a close shot shows that he still has the hi-tech weapon in one hand, and only then he drops the weapon.
When Juve falls out of the plane near the end of the movie, he is very obviously played by a stunt double; moreover, the double wears badly concealed parachute packs on back and belly.
The music in the ballroom scene doesn't match the instruments we can see in the band.
Juve is kidnapped while wearing his pirate costume. Imprisoned, he's seen without the fake peg leg, but wearing two matching shoes. As the costume had one peg leg, it's unlikely (but possible) it came with two matching pirate shoes, or that the kidnapper was kind enough to find one and provide it.
The sign "XIV congresso scentifico internazionale" incorrectly spells "scientifico"