When Donnie is about to enter the car with Gretchen near the end of the film, he clutches at his stomach as if in pain. Near the end of the film, we see Frank touch his eye after Donnie dies. Both of these are forward memories of pain: one of the deleted scenes features how Donnie dies - a pole goes through his stomach right where he clutches it. And where does Donnie shoot Frank? All together now - the eye.

The time when the world is supposed to end is 28 days, 6 hours, 12 minutes, and 42 seconds. 28+6+12+42=88. The year the film takes place is 1988.

Frank tells Donnie that the world will end in 28 days, 6 hours, 42 minutes and 12 seconds. The jet engine is shown landing in Donnie's room, killing him, exactly one hour, 42 minutes and 12 seconds into the movie.

The number 88 may also be a small reference to 'Back to the Future'. Not only does Donnie mention the DeLorean while talking about time travel, but 88mph is the speed the DeLorean needs to reach in order to time travel in BTTF.

In the "Cunning Visions" infomercial, Jim Cunningham (the man arrested for having kiddie porn) grabs the young boy actors behind.

When Donnie is riding his bike at the beginning of the movie, the Trans Am with which Frank runs over Gretchen is seen during this opening sequence.

Some of the people in Jim Cunningham's video make cameo appearances at Donnie's school. The little boy on the video is the same kid that asked "how can I learn to fight" at the assembly in the auditorium. Also, the older girl in the video is the same girl who shows the newspaper with Jim Cunningham's arrest to Miss Farmer in the High School's main office. Their presence and/or placement at the school seems to indicate that Jim Cunningham's entire "Q + A" seminar at the school was staged, including the questions to be asked of him. This would also prove Donnie right about Jim Cunningham being a fake, only pretending to help people, as well as explaining his rapid success which is evidenced by his large home.

After taking lessons to prepare for her role in the film, Tiler Peck (Beth Farmer of Sparkle Motion group) pursued more lessons after the film, and is now a soloist with the New York City Ballet.

In the scene when the English Teacher is packing to leave her job, and Donnie asks about the meaning of 'Cellar Door' written on the board. She responds 'A famous Linguist once found those were the 2 most beautiful words in the English Language'. The Linguist she was referring to: J.R.R. Tolkien, (author of 'The Lord of The Rings' trilogy).

Jason Schwartzman was originally cast as Donnie

Both Vince Vaughn and Mark Wahlberg were lined up to play Donnie. Vaughn refused because he didn't feel comfortable playing a 16 year-old. Wahlberg wanted to play Donnie with a lisp but the director disagreed.