Revealing mistake: When Chuck is writing on the rock waiting for the wind to change direction, you can see that when the wind changes direction the trees behind him are duplicates.
Revealing mistake: As Tom Hanks escapes from the sinking aircraft, it shows him still underwater, looking back and the plane breaking apart. It shows an AKE-shaped container (which fits the contour of the bottom of the aircraft) which has no markings at all, it appears to be just a computer generated solid with no color. (00:27:00)
Revealing mistake: The container that breaks the net is made up to be an AMJ container, with the 45 degree angle on the upper corner to fit the contour of the plane. However, as the camera zooms back out to show Tom Hanks, you can see that the side of the container towards the center of the plane is entirely clear, with packages visible. If it was a real AMJ, this side should have a white or purple curtain covering almost the entire side. That can is a prop. (00:25:45)
Revealing mistake: When Chuck is lying on the raft, and sees the giant cargo ship that rescues him, there is a closer shot of him and the raft as he stretches his hand out in order to be noticed by the ship's crew. He's supposed to be out in the middle of the deep ocean, but the water below him is pale green and very shallow. You can even see the bottom (hard to notice but it is there). It's obviously the water tank they were filming that scene in.
Revealing mistake: When Kelly is copying her dissertation, there is no paper being fed through the copier feeder or any printed pages going into the output tray.
Answer: No one knows what was in the package. I think Chuck doesn't open it in order to keep his sanity. The package is his only link to his life before being stranded, working for FedEx and making sure packages get delivered. He was determined to make sure it got delivered just as he was determined to survive and get back to civilization. Success in one meant success in the other. While Zemeckis has joked there was a waterproof satellite phone in it, the real answer from the script is, no joke, salsa verde. Its a care package with a note imploring Bettina's (the artist) husband (naked cowboy) to come back and spice up their life like the salsa.