Shown in the bar mirror, Rawhide puts her arm over Neal's arm when she sets her drink down. When the angle changes to the actual actors, their arms are nowhere near each other.
The brim of Norman's hat changes shape several times while fishing with Paul after his return to Montana. The reels also change on his fly rod.
Young Paul has brown eyes in the film, while the grown up Paul has blue eyes.
When grown-up Paul catches the biggest fish ever and is washed downstream, the fish differs between a salmon and a large trout between scenes. The "tell" is the nose of the fish.
When leaving the gaming house, the car Norman is driving has the passenger side lamp out. After Paul talks to Norman and the camera pans back, the headlamp is back on.
Norman is shown as a middle-aged man when he hears his father delivering a sermon shortly before his death. But Norman was only 39 when his father died, which was only three years after Paul's murder.
Obvious stunt double for Paul when he is being dragged down the rapids while reeling in the big fish. The double's hair is dark brown/black under the hat, while Brad Pitt has blonde hair.
The scene where Paul is fighting the big fish his hat is on while being submerged, going down stream through rough water and finally landing the fish. However, in the following immediate scene when he poses for Norman to take a picture of him holding the big fish his hat is missing.
Paul tells his family the story of how he went fly fishing with President Calvin Coolidge. But he never mentions the widely reported faux pas that Coolidge committed and which, Paul, as a dedicated fly-fisherman, would have noticed: that the President used worms instead of flies as bait. More than three decades later, in fact, Newsweek reported that "the fishermen of America never got over the fact that Coolidge angled for trout with worms instead of flies."
When Paul holds up the big fish, there is some monofilament line visible that goes through the fish's mouth.
While on the library steps, Norman speaks of meeting boxer John L. Sullivan while at Dartmouth. Norman's departure for college was 1919, yet Sullivan died in 1918.
When Norman goes to Dartmouth, a modern-day truck is visible in the background.
After Neil boards the train for Hollywood, there is a modern semi and trailer on the distant highway left of the train car.
Paul does a Bogart imitation, while at the speakeasy. The scene takes place in 1925 during the silent movie era, so he would not have even known what Bogart's voice sounded like. Bogart was an unknown in movies until the 1930s.
The Burma Shave saying on the library steps was not used until the 1930s. However, this part of the movie was set in the 1920s.
At the Labor Day picnic, Reverend Maclean is playing horseshoes when Paul arrives. Paul addresses his father by saying "Hello, Father." Although you can see Reverend Maclean say "Paul", the dubbed voice says "Son".
During the shooting of the rapids, cables can be seen overhead of the actors in one of the shots