Visible crew/equipment: In the scene at the travel agents when Truman is trying to book a flight to Fiji the woman asks 'when would you like to leave?', Truman holds up his suitcase and says 'today'. When he holds up his suitcase you can see the boom mic reflected on his suitcase. (00:41:25)
Continuity mistake: In the travel agency, some pencils appear, others disappear, and others change position during the whole scene. (00:43:00)
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. (00:48:00)
Continuity mistake: When Truman and Meryl are discussing in the kitchen, she threatens him with 3 kitchen gadgets. Depending on the shot, the position of the gadget in the middle keeps swapping from being equidistant from the others, or close to the one on the right. (00:54:15)
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. (01:18:25)
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. (01:19:05 - 01:25:25)
Continuity mistake: In the boat after the storm, Truman has a rope going over his thigh. A second later it is tied around by a big knot. (01:28:15)
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. (01:32:15)
Continuity mistake: At the end, when Truman is supposedly looking into the sun, there is a shadow on one side of his face, indicating that the light source for that shot is to his side. (01:32:15)
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.
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.
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: During the scene following the leak at the plant when Truman's wife is trying to ask him what is going on, she is standing next to a counter. Look at the farthest left door. A shadow of a crew member can be seen moving.
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.
Other mistake: After Truman opens the door to the outside world there is a close up shot of him listening to the creator. Behind Truman is a solid black wall, not the shadowy space we would expect and do find in later scenes.
Continuity mistake: The FM LED on the car's radio appears in the outside shots but disappears when seen from the inside of the panel.
Continuity mistake: Truman's Ford Taurus is brown in color. Yet, when he tries to escape with his wife the car is silver for that entire scene.
Continuity mistake: In the flight centre, after Truman taps his luggage, he puts both his arms under his chin. Then the camera changes shot to just beside his elbow, and his fist is sticking out behind his elbow. Just a shot ago it was below his chin.
Visible crew/equipment: Right as Truman is leaving to go to Fiji in the sedan, he backs out of the driveway. The shot is a camera attached to the rear of the vehicle. The rig is reflected in the bumper on both sides of the license plate.
Revealing mistake: If you look in Truman's car's mirror when he is backing up from the traffic jam, wires can be seen wrapped around his car.
Chosen answer: Clearly, in his media interviews, Truman's best friend, Marion, indicates that he did, indeed, grow up with Truman since they met in elementary school. One could extrapolate that his classmates did, as well. A sudden change of actor from one day to the next would be too much of tip-off to Truman. As actors leave the show, there is a story along with it. His father is presumed to have died, and returns later. I'm sure others move away. The main characters (Truman's wife, parents, close co-workers, and other key figures in his life) also appear to live "in world" and have devoted their entire lives to the Truman project. It is not made clear about the extras, though.
Michael Albert
It's kinda hard to believe though that a little boy (Marion) is faking it all so well for all these years since his childhood without raising any suspicion.
He probably didn't get to know it until a late age. In the bus, a little girl was confused and almost tell her mum that there was Truman, so a kid can't keep up the secret. I'm pretty sure they don't know it. But on the other hand, when he says he wants to be an explorer all the classmates freak out, as if they knew it would put in danger the show. So maybe the classmates and even the best friend had been selected after making sure they wouldn't mess up and reveal the secret.