Editing is never random.

Rating: 9/10 (20 January 2005)

**minor spoilers**
---Pieces are shown a number of times with no real linear link between them, just like it would be if we ourselves had a memory disorder, and then they are cut up and edited next to things that happen either before or after it.----
The sequencing is actually another part of Memento's masterful construction. The first scene, during the credits, is the only scene in which the action unfolds in reverse. This both sets up the momentum of the film, and sets up the "fragility" theme--part of Lenny's existential crisis is that his actions may not be meaningful if the evidence of them, the photos he takes as mementos, can be easily destroyed. Since he is unable to make a memory, he doesn't *remember* anything when he sees the photos, thus they are not *reminders* but *mementos* whose meanings can easily be altered or destroyed entirely. Lenny bases all his actions on his mementos. He chooses to destroy Teddy's photo because that effectively "erases" the action of his murder from Lenny's archive of mementos.
But I digress...
The first scene actually unfolds in reverse motion. Subsequently, all of the color scenes take place in reverse chronological order. The intercut black and white scenes occur in forwards chronological order. The last scene of the movie is actually the middle of the story; the events unfold both backwards and forward until they collide in the middle, at the moment of Teddy's murder. To use the alphabet as an example, after the credits scene, the story unfolds in this sequence: Z, A, Y, B, X, C.... with Z, Y, X... being in black and white and A, B, C... being in color.
It's the classic film noir flashback on acid.