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 Kitano brings an umbrela 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.
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)
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.
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 its 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.
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.
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.
The moment Nobu's neck collar explodes there is a shot of him falling face first onto the class floor. In the background you can see Lieutenant Anjo standing upright near Kitano and not flinching. Afterwards Shuya kneels down and cries for him and you can see Anjo bent down and slowly stand up like he was defending himself from the explosion.