When R.C. wakes up the first morning in the tree, he sees a bird's nest and takes an egg from the nest. He uses his knife to crack the egg near the top end. In the closeup, it shows a larger egg that is cracked in the middle.
When Robinson looks at a neighboring island for the first time through his telescope, the scene shown as though looking through the telescope is just a picture of the island - nothing in it moves including the waves.
When RC first notices a human footprint in the sand on his island, it is a single footprint, with no other footsteps or any means of having walked away.
Two different cats are used that look nothing alike to play the same cat. Sam, the cat he rescues from the shipwreck is a calico. By the time they reach shore, Sam has somehow transformed into a gray and white tabby and remains that way throughout the rest of the movie.