Continuity mistake: After Chuck Nolan (Tom Hanks) injures his hand and throws the volleyball "Wilson", in the following scenes pay close attention to the box that the volleyball is in. One scene the box has a visible tear along the top rim of the box; then, before Tom Hanks opens the box to remove "Wilson" another box appears not torn.
Continuity mistake: On the island, immediately after the title indicating "4 Years Later" Tom spears a fish. The spear hits the fish hard and swiftly, and as the camera pulls away the spear is very close to vertical. For the spear to hit that hard and vertical it had to be thrown close to the fish and almost straight down. But as the camera continues to pull further away, you see Tom standing too far away to have thrown the spear in that way.
Continuity mistake: During the scene where he is first at the cave, look in the sky. First, there aren't any clouds then there are in the next scene. If you keep watching, the clouds vary from the different camera angles.
Continuity mistake: When Tom Hanks sees the ship from the beach, it is obvious there is a pane in front of the camera. However, as he is jumping up and down through the scene, his shadow changes angles from in front of him, to a very low angle behind him.
Answer: Cast Away was filmed on an island called Monuriki. It's part of a group of islands called the Mamanuca Islands, in Fiji.