Factual error: At the start of the excommunication scene, the monks are chanting Dies Irae, a thirteenth century Catholic poem, but St. Thomas Becket died in 1170, almost a century before the poem was written.
Visible crew/equipment: The shadow of a camera and cameraman are revealed on the left side while the President (Frederic March) is moving from one room to another (door jamb), after learning about Martin Balsam's death. (01:20:00 - 01:21:00)
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.
Continuity mistake: The three enter a small cantina and start a fight and fire. The place is huge on the inside, with stairs to a second floor. From the outside it's just a small shack.
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.
Factual error: In a scene set in 1871, several newspaper articles get printed on Moe's face as he falls onto a printing press. Curly Joe reads that "AT&T is splitting two for one." American Telephone and Telegraph Company was founded in 1885.
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)
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.
Visible crew/equipment: A camera shadow is visible on the detective as he bends down to inspect Clara's body.
Factual error: The radio Walter uses on the island has no apparent power source; we never see either batteries or a hand-cranked generator.