The first time the inside of Sebastian's journal is shown, "The Bet" page is opposite the page with Kathryn's picture and the phrase "I'm a bitch". At the end of the movie, Kathryn receives a copy of the journal and opens to "The Bet" page which is now opposite Cecile's page.
In the closing scene Kathryn looks up at the students after skimming through the pages of Sebastian's journal and they are all looking at her. They are all standing in a shaded area with absolutely no sunlight on them. Just a second later there's a closeup shot of one of the same students and there's sunlight on his face.
In the luncheon scene in Aunt Helen's yard, she leans down to hug Sebastian after saying the line, "You two are the best!" In one shot, she moves her left hand to his left shoulder, but in the next shot (close-up) her hand is not on his shoulder.
When Annette meets up with Sebastian to have a swim the towel she places on the rail is black but when she gets out of the pool she grabs something white which is assumed to be the towel. In fact, the white thing is her bathrobe; before the swim, she places the black towel on the rail and takes off her robe, which is also placed on the same rail .
When Sebastian drives home after going to lunch with Clorissa, he makes a U-turn which forces the taxis behind to stop. However, they start honking when his car goes right - this was obviously planned because there was no reason to assume he would U-turn.
The goof items below may give away important plot points.
When Kathryn is in the bathroom before giving Sebastian's eulogy she is wearing two rings - one on each middle finger. After she walks out of the church and Cecile gives her the copy of Sebastion's notebook she is no longer wearing any rings. Then, just a moment later after the headmaster takes her hand to find the cocaine in her crucifix she is wearing a ring on her middle finger again.
In the closing scene as Annette is driving out of the city there are a few shots from the car (side on) where you can see sand and hills. Then it cuts to a front shot of the city, with no hills or sand in sight.
When the taxi hits Sebastian the stopping distance of the car is extremely long for it's speed. In reality from the speed of 40-50 Km/h the car should stop in a few meters presumably not hitting Sebastian. That's because the stunt was made with the car traveling at a modest speed and with almost no braking. Obviously the screeching tire braking sound was added later fact confirmed also by the nonexistence of the braking marks on the road.