Revealing mistake: When Dormer fails to pursue Finch during the log chase scene, he climbs out of the water by a ladder. If you look at Dormer's back as he gets out of the water onto the dock you can see an outline of the life jacket Pacino wore to film the scene. You can see the outline again when he turns around and walks up the walkway. (01:00:49 - 01:01:11)

Insomnia (2002)
1 revealing mistake
Directed by: Christopher Nolan
Starring: Al Pacino, Hilary Swank, Martin Donovan, Oliver 'Ole' Zemen, Robin Williams
Continuity mistake: In the scene on the ferry, when Williams and Pacino are talking, in the shots showing Williams, Pacino is facing the center of the ferry frequently, while immediately afterward in the shots showing Pacino, he is facing Williams again.
Walter Finch: You forgot the wild card, Will.
Trivia: Al Pacino's character is named Will Dormer; Dorm is the latin root for 'sleep'.
More questions & answers from Insomnia
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Chosen answer: It represents Al Pacino's character planting evidence on men he knew were guilty. He was planting the victim's blood on their clothes so the evidence would be stronger against them.
In addition, there is a repeated scene where someone (Dormer) is wiping something on or off a fabric.