While Mackelway is reading his first fax, some of the pieces of paper on the right side of the bulletin board appear and disappear between shots.
As Agent Mackelway follows the patient that asked him about the 50-foot fish, he, Mackelway, puts some paper into his left jacket pocket. When the shot changes, he's holding the paper in his right hand. At the next shot, the paper is gone.
The trailer of the big rig that crashes at the end has several damaged areas on it that were not there prior to the crash. (Possibly from an earlier take that didn't go right and damaged the trailer.)
There is a spelling error of the word 'condensed' (spelled condenced) on O'Ryan's FBI psychological evaluation when Mackelway is reading it
(at around 27 mins) Mackleway is looking at a newspaper clipping dated Friday, October 23, 2001. Tuesday is the actual day of the week for that date.
(at around 1 min) The agents are trying to pinpoint missing people on a map with locations marked with yellow push-pins. One of the people missing was from Dyersville, Iowa which is strange for 2 reasons: 1. the dot is located in the south-central portion of the state hinting that it may be the capital Des Moines and 2. Dyersville is in the far eastern part of the state while the lead agent points to eastern Nebraska.
When a Missing Person report comes over the Fax, they misspell Wichita, Kansas, adding an extra 't'.