When Dr. Cammon explains the legend of Hercules, she says that his sword cauterized the hydra's severed heads to keep them from regenerating. Legends actually say he used a sickle or club to take off the heads, and his nephew Iolaus used a torch to cauterize the stumps.
Dixie is grabbed by the hydra and pulled down the corridor. A few scenes later, a shot is shown of the place where she was grabbed, with a puddle of blood and a missing shoe, neither of which were in the original shot.
Dixie and Vincent are tied up by a lifeboat when the hydra boards the ship. The camera cuts to a different scene. When we next see the lifeboat, there are bloodstained hand-prints on it and shredded ropes. A few seconds later, though, both Dixie and Vincent are shown running around the ship with no blood on their hands.
Valerie was carrying a lantern and a backpack. When she ran out of the cave, she had no lantern or backpack. Then, in the next scene, her backpack is back. But the lantern is still gone, presumably, she may have dropped it before running out of the cave.