Other mistake: The impression of Robin Hood's hands in cement, made as a child, match the size of his adult hands.
Suggested correction: There were two sets of handprints. The one on the left was that of a child. The one on the right was that of an adult. When Robin reopens the slab, the prints are covered in mud and debris. He runs his hand over the child print then puts his hand in the adult print of his dead father.