Mosaic in the floor of the entrance way.
Apparently Flagler was the son of a minister. When building the hotel he went to great lengths to make sure that everything was perfect. However his religious background told him that only God could make things perfect so therefore he had the person making the mosaic include a flaw in the pattern. Around this central large dark circle there are a series of black triangles. If you look at the one at the top of the picture you will notice it does not have a small white piece at the bottom. This was Flagler's built in flaw.