Things that the cane may not detect, even when properly used in constant-contact cane technique, are:
Small objects or holes, which the cane may go around even when sliding on the floor in constant contact technique – see photos
Poles and corners which the cane may go around (if the pole or corner is close enough to the cane hand, the shaft of the cane will contact it, otherwise no part of the cane will contact it and the person may bump into the pole) – see photos
Overhanging obstacles, such as tree branches, and signs or objects that protrude from a wall or pole – see photos