Continuity mistake: When Shirley Maclaine goes swimming in the blue lagoon, her hair is all wet before she enters the cave, but once in the cave her hair is only wet on the ends.
Continuity mistake: At the ring toss booth, there are five rings on the bench as one of the booth attendants is counting coins from the till. When the other booth attendant turns to give Leo the doll he won, the rings disappear, then reappear.
Factual error: In reality, Commodus did not die in the arena - he was strangled in his apartments in 192 A.D. by an athlete who was hired by a group of Senators conspiring against him.
Audio problem: When the Big Bad Wolf is blasted through the wall a second time, we cut back to Bugs who says "I don't see why anybody thinks these club initiations are dangerous." When he says it, the audio is a few frames behind his mouth.
Continuity mistake: When Peter Fonda is hanging from a cliff with both hands over a waterfall, he is wearing his glasses. There is a cut to a shot of him not wearing glasses. When the movie cuts back to him shortly afterwards, he is wearing his glasses again. (00:32:10)
Factual error: The radio Walter uses on the island has no apparent power source; we never see either batteries or a hand-cranked generator.
Other mistake: There were no gorgons named Megara. The three that existed were Medusa, Euryale, and Stheno.
Continuity mistake: When we see a close-up of the meter, there is a box saying "10 Centavos for One Hour." The words were not in the box in the previous shot.
Continuity mistake: When the clerk looks in close-up at the bank cheque written in disappearing ink by Fantomas, the background has nothing to do with the desk, and the distance is all wrong as well. (00:03:10)