(at around 12 mins) When Kitano throws the knife and hits the first girl to die we see the knife land in the middle of her forehead, yet when he kneels down to retrieve the knife he pulls it out of her eye. (In later versions of the film this error was digitally fixed)
(at around 1h 14 mins) When Yuka Nakagawa is poisoned, her face lands in the bowl of poisoned spaghetti. A little later as the girls begin shooting each other, her face is now out of the bowl, and on the table instead.
(at around 56 mins) Before Kawada reveals the picture of him and Keiko, we can see its outline from behind, and it shows a different couple.
(at around 41 mins) Hirono's weapon changes from being a Browning High Power to an Colt .45 Model 1911 (just after she throws Mitsuko's weapon).
(at around 52 mins) After Chigusa stabs Niida repeatedly, she gets up and finds Mitsuko standing distantly behind her. In the same shot, Mitsuko smiles at Chigusa, but when the camera cuts to Mitsuko, she smiles again.
(at around 1h 12 mins) In the lighthouse, Yuko Sakaki accidentally poisons Yuka Nakagawa with powder from a bottle labeled "KCN" (potassium cyanide). About 10 seconds later, Yuka vomits blood (which suggests internal bleeding) and dies. Cyanide is actually a poison that shuts down the respiration in cells. Ingestion of more than 50mg of cyanide will result in rapid respirations, loss of consciousness, convulsions, gasping, and death due to cessation of respiration.
Bags that are given to students aren't big enough to fit some of the weapons (e.g. crossbow, sickle, etc).
A rolling, decapitated head still wearing a helmet strapped to his head.
(at around 39 mins) During the second day, the movie lists the time 0:00 PM, which is a time that neither makes sense nor does it exist. The school teacher then states "It's noon" and lists the danger zones "an hour from now at 1300". The time should have simply been stated as 1200, and if it's military time it does not need to say AM or PM. However, in Japanese, time is usually counted from 0:00 to 11:59, with 0:00 PM being noon and 0:00 AM being midnight.
(at around 8 mins) At the beginning of the movie, the class acts shocked to see Kitano greet them at the island. He says he is taking over the class. However at the end of the movie, you see a class photo with Kitano as their teacher.
If it's not apparent in all versions, Kitano was not respected by the students and left the class in 7th grade. While some students may suspect what his return means, others fall back to disrespect and challenge him.
Note that the class photo (with Kitano) is also shown in the beginning of the film, but the "extra students" are only inserted in the last usage.
When characters stab or shoot each other through clothing, there are bloodstains but no holes where the bullets or knives go through.
(at around 1h 16 mins) Utsumi is breathing when Shuya finds her body in the lighthouse.
(at around 51 mins) When Chigusa attacks Niida, she stabs him in the back, they roll to the floor, and then he tries to run, but she grabs him and screams "Bastard!" However her mouth is shut.
(at around 56 mins) When Noriko tells Kawada that the food that he cooked was good, we hear him reply that his father was a cook even though at that moment his mouth doesn't move.
(at around 4 mins) When Noriko is on the bus with the others and is about to go give the cookies to Shuya & Nobu, she is pulled through the aisle by Negumi who yells to everyone in front of them to get out of the way but her mouth doesn't move.
When the lady on the video is calling out the student's names Girl 1 (Inada's) response of "here" can be seen displayed on the screen before she can actually be heard responding to her name.
Given that the events in the beginning of the film shows the winning student from previous year with rather intense media coverage, the students in the competition depicted in the film should have been aware of the existence of the competition and what was at stake before it was explained to them.
Some of the kids are automatically gun experts even though they have never shot a gun before.
(at around 1h 23 mins) When Kitano brings an umbrella to Noriko in the forest you can see the mic just above Noriko's left shoulder, it tries to hide itself with the actress's body but fails. You can easily identify it with the fur cover on it to cut down wind.
At the start of the film, Nanahara states that his father 'hung' himself. The word 'hanged' should be used when describing someone who commits suicide in this manner.