There's a moment around Episode 8, the beach trip, where everyone gets tense just as they're leaving the Yomiyama city limits and a big rig is passing them. A reasonable move on their part, up until the reveal of the final episode. There was very little chance that the big rig would cause some kind of accident at that particular moment because the "Extra" was there in the car with them.
The final episode and the OVA reveals that Fujioka Misaki's hair is brown, not black like her sister's...which means the girl at the end of the ED is Fujioka Misaki, not Misaki Mei.
Why can't Kouichi remember being in Yomiyama or meeting Izumi? Perhaps he was there for his aunt's, Reiko, funeral.
Aunt Reiko's dislike of the mynah bird. It's revealed that she died a year and a half ago and her parents bought the bird as a Replacement Goldfish. The words the bird keeps repeating "Why, Rei?" and "Cheer up" are what the grandparents said after the funeral. Aunt Reiko hates the bird because it subconsciously reminds her that she's really dead.
The anime title is in special brackets that a mathematically inclined viewer may recognize as a ceiling bracket and a floor bracket, which mean to round up and round down respectively. This means the brackets around the title can be read as saying "round up in the beginning, round down in the end" which is a nod to the premise of the series. This may not have been intentional however, considering how often the same brackets show up in other anime.
Why do people in and related to the class all start dying suddenly? The world somehow knows that the "class that invites death" resurrects someone, and it's nature's way of trying to return the world to the way it should be—by killing the person who is supposed to be dead. For some reason, though, the world's worse at figuring out who's the dead one than the people in the class, and simply kills people at random in the hopes of eventually getting it right.
The Un-Person tactic adopted by the students to protect themselves from the curse is noted to have only a partial success rate to start with. In this particular instance, however, there's actually a specific reason it failed: The Extra this year was never one of the students, so using the Un-Person trick to ignore one student and 'even out' the numbers was doomed to fail that time around.
Mei and Kouichi seem to get a free pass when it comes to the curse. Yes, both of them are put in danger, and narrowly avoid two separate encounters with falling glass. And yet they are the only ones we know of that actually SURVIVE brushes with the curse (if indeed it was the curse). Moreover, in the last few episodes they are threatened with death directly by a number of students. Those that come the closest to killing them suffer fairly improbable deaths - falling out a window, accidentally hanging oneself from stray cables, and being impaled on glass shards after a lightning strike causes a window to burst open. The curse almost seems to be protecting them both by this point! Perhaps it has a fondness for those the class declares "a person who does not exist"?
They've had 26 years of unfortunate practice, after all.
We learn that there's a way to stop the curse after it starts up, but making use of it requires being able to identify the "Extra" (impossible in normal circumstances). It also RetGones everyone's memories of the "Extra's" existence. Even the person who pulls it off ultimately lacks a Ripple-Effect-Proof Memory. So you have a method of stopping the curse that is not only unreliable in and of itself but can not be reliably passed down to future generations, making it unlikely the curse could ever permanently be stopped. Even if the above weren't true and students were able to consistently stop the curse, the memory factor means that overuse could end up causing students to forget the curse altogether, starting the entire cycle back at square one.