Factual error: When Hooper sees the hole in the hull of Ben Gardner's boat, he uses his knife to pry out the shark tooth. The tooth is located at the bottom of the hole, with its flat root side stuck deep in the wood and its pointy side facing up. It is completely impossible for the shark's tooth to become wedged in the wood this way, while he takes a nice bite out of the wood hull. (00:49:15)
Character mistake: Captain Quint tells Hooper and Brody that the USS Indianapolis delivered "the Hiroshima bomb" in 1945. This is a myth that has persisted for decades and right up to the present. To prevent it from being lost in one piece, the Hiroshima bomb was actually delivered in pieces by various means to Tinian Island, where the parts were reassembled before it was carried aboard the Enola Gay to its target in Japan. The USS Indianapolis delivered only the detonator for the bomb. (01:29:00)
Character mistake: Chief Brody and Hooper go to the wharf to dissect a large tiger shark and examine the contents of its stomach. Finding nothing unusual, Hooper recommends they go offshore that night to search for the real killer shark because "he's a night feeder." Coming from a marine biologist, that remark really makes no sense. Hooper knew that, in addition to eating Chrissie the midnight swimmer, the shark also ate Pippin (the black Labrador retriever) and the Kintner boy in the middle of the day at a public beach. Based on all available evidence, the shark was no more likely to feed at night than in broad daylight.