Well, you have to watch or have already seen the movie to catch this. It's really weird because the movie plays backwards. Like, the stuff that happened today would be the first thing you see, and the event that happened two months ago leading up to today would be the last thing you see. Throughout the movie, the guy gets tattoos so that he doesn't forget what has happened. He has to do this because he has a condition that doesn't allow him to make new memories. So he gets these tattoos, and they are numbered 1-6. So near the end of the movie, which is really the beginning of all the events, he gets another tattoo, and it is tattoo number six. But since the movie plays backwards and that was really the beginning of all the events, it should have been number one.