Early on in the movie, Kyle's computer monitor changes from a bulky white one to a thin black one. Later on in the movie, Lance buys him the black monitor.
The footprints Kyle leaves on the glovebox in his dad's car change position when his dad looks at them again after Kyle's death.
When Lance, Kyle, and Claire are sitting on the table, they are equidistant and 120 degrees to each other. When camera turns to Kyle POV under the table, it is directly opposite to Claire.
After Kyle's journal is published, the motivational sign over the blackboard in Mike's classroom reads "Hard work is it's own reward". There is no apostrophe in "its" when it is used as a possessive. Mike teaches creative writing.
When Kyle claims his friend Andrew has asthma, Lance replies that Andrew should not be having milk products; the notion that people with asthma should avoid dairy has long been debunked by medical experts.
At about the 1:03 mark, the bulletin board in the background at MacDonald High School reads "MACDONALD STUDENT'S ARE...". The apostrophe should not be used in the word "students" when it is the subject of a sentence.
When Heather taps her ring on Lance's car window to ask him for something of Kyle's, she taps three times but four taps are heard.
When Lance dives in the pool naked, you can see in the air bubbles a pair of shorts on the stunt double just after he enters the water that were cleverly edited out later in the scene.
When Mike is reading the coroner's report, where it says manner of death, "natural" is checked off instead of "suicide".