Revealing mistake: After Bluto knocks Popeye out and he falls into the water, Bluto goes to jump in after him. Watch the water. Popeye just landed, but the water is really calm. And when Bluto hits the water, it's right on top of a face-down floating Popeye. As Bluto hits, Popeye's body bends almost in half backwards, revealing that it's a foam rubber mannequin.
Revealing mistake: At the end, in the fight with Bluto, Popeye was facing him on one side, goes underwater, and pops up on the other side. If you watch closely, the figure that goes under is the stunt double, as nobody can get from one spot to another about 15 to 20 feet away underwater in the space of three seconds. Also before Popeye comes up, you can see him underwater as the other figure goes under.