The movie is set in the month of January. Within the movie, they walk around as though it is hot outside. This would not be the case as the average January temperature in Tokyo is approximately 43 degrees Fahrenheit (6 degrees Celsius).
The movie takes place in January 2003 yet the characters make several references to "YouTube". YouTube was not created until February 14, 2005, more than two years after the movie takes place.
Many posters can be seen for movies that wouldn't be released until 2007, despite the movie taking place in 2003,
There are scenes at the start of the film which are obviously filmed in Tokyo (in Shinjuku) but the caption giving the location is clearly not Shinjuku Gyoen as there is no giant wooden torii gate there.