"In the second half of the film, Frodo has a scar on his lower right cheek, close to his chin. Many times throughout the rest of the film the scar changes position and size on his right cheek. It also appears on his left cheek in flipped shots (most obviously on the slopes of Mount Doom when Sam is cradling his head). (02:31:05 - 02:34:00)"
And don't forget the "Speedo tan" in the waterfall scene...