Donnie: Life isn't that simple. I mean who cares if Ling Ling returns the wallet and keeps the money? It has nothing to do with either fear or love.
Kitty Farmer: Fear and love are the deepest of human emotions.
Donnie: Okay. But you're not listening to me. There are other things that need to be taken into account here. Like the whole spectrum of human emotion. You can't just lump everything into these two categories and then just deny everything else!
When Cherita makes her mark on the lifeline blackboard, there are some smudges on the broad chalk line. In the next shot, the line is perfect again.
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.