Continuity mistake: When Marlon stocks the vending machine, the camera (inside the back of the machine) shows him stocking the 1st row with brown-wrapper candy bars. He starts stocking the 2nd row with yellow-wrapper bars; Truman interrupts him for a minute (during which Marlon doesn't touch the bars at all). When Marlon continues, the yellow bars have changed to something darker (it's not just a different camera view; even barcodes are different). After another brief interruption, Marlon continues again, and the yellow bars are back.
Continuity mistake: When Truman is going mental, with his wife in his car and they are about to go around the circle loads of times, it shows a shot of the car from the back. If you look at his wife's seatbelt it is on. However, a couple of seconds later it has another rear shot of the car and you can see the wife putting on her seatbelt again, when she already had it on.
Continuity mistake: In the scene where Truman is on the boat, it shows a shot of him wearing a green shirt and black trousers but in the next shot he is wearing a blue shirt and khaki trousers.
Visible crew/equipment: When Truman takes his high school sweetheart to the beach, her "father" comes to get her. There is a shot through the father's button cam showing Truman backing away. Look at Truman's shirt and you will see not one but two human shadows. One is the father and the other is the "button" cameraman.
Factual error: When the bus breaks down, steam is shown coming out of the front of the bus. GMC buses have the engine and radiator in the rear.
Audio problem: When the entire city is searching for Truman near the end of the movie, you hear everyone as if they are in step with military-like cadence. However, take a look at the group and you will see they're not even close to being in step.
Other mistake: When Truman is standing at the open doorway at the very end, and is addressed directly by Cristof, during some of the shots, the open doorway behind him becomes a black backing board, visible from the way the light falls on it.
Revealing mistake: When Truman is first discovered escaping on the boat, they go to a camera shot of his face. If you look at the water, the boat is not moving at all. They use a similar shot later in the scene after the storm. You can also tell the background is the same as the large wall that Truman crashes his boat into.
Continuity mistake: When Truman talks to Marlon, who is stocking the vending machine in the shop, Marlon's head jumps from looking at Truman to looking inside the machine, placing the bar, in a single cut.
Continuity mistake: When Truman is on the bus to go to Chicago, he climbs on, and the door shuts. The bus driver screws up the motor, and when we flash back, the door is wide open.
Continuity mistake: When the fallen lamp has settled, the glass and parts of the housing are in the sunlight, and the light is in the shadow, with the camera looking into the side of the shadow. As Truman comes out to investigate, the light has been turned to a new direction, the pieces of the housing are missing, and the glass is splayed out a lot more than it originally was.