When it begins to rain on Truman on the beach, the single spray head follows him as he runs around. It would take at least 8 or 10 seconds for the rain to reach Truman, assuming the shower heads were up in the sky far enough to not be visible. The shower heads follow him too quickly and appear to be only a few feet above the view of the camera
When the neighbors are walking arm and arm in search lines, the audio is of footsteps marching in perfect synchronization. However, the people's heads are bobbing up and down out of synchronization, and when their feet are shown, they are walking out of step.
It is an enduring myth that the Great Wall of China is the only man-made object visible from space. The wall is narrower than the average highway and is visible only in low orbit from where all manner of other man-made objects can also be seen.
When Truman notices Meryl's crossing her fingers in the wedding photo, the hand that is visible would be her right hand as she is standing on the right side of the photo looking at Truman who is on the left. The mistake is that Meryl's wedding ring is on the hand with the crossed fingers, which would be her right hand. (This may agree with the "lie" of the crossed fingers.)
Truman states that Fiji is exactly on the other side of the Earth from Seahaven. ("You can't get any further away before you start coming back."). In fact, the furthest point from Seahaven on the earth is in the Indian ocean, about 2000 miles off the coast of Madagascar.
When Meryl gets in the car with Truman and he drives off to the traffic circle, the above shot shows Meryl wearing her seat belt. A few seconds later as they are driving around the circle, the camera shot from within the car shows Meryl putting her seat belt on.
When Marlon is restocking the shelves he finishes the row with brown candy bars...takes two out of that row and turns to talk to Truman. When the camera angle goes back to the view from the inside of the vending machine, Marlon restocks the first row with three brown candy bars, he didn't have any other bars in his hands when he removed the two earlier.
Truman's Ford Taurus is actually three different models. The GL, the LX and the performance edition SHO. The models change frequently in the film. The wheels, interior trim, and exterior styling all have subtle (but noticeable) differences.
In the "Moon" room just before Truman's escape from the basement, Cristof enters the room and to the left we see a reflection of one of his employees sitting in a chair. Unfortunately, that employee has already left the chair in the prior shot.
In most of the interior shots of Truman's car, the Ford Taurus has manual air conditioning. However, as he parks up outside his office, the close-up on the radio shows a model with electronic climate control.
When Marlon is restocking the candy bars in one of the vending machines, he works on filling up one of the rows. The scene switches to Truman talking with Marlon, and when the scene switches back to the vending machine, the row that Marlon had filled in is empty.
When Truman makes Marlon leave work after stocking the vending machine there is no box full of candy. When Marlon was stocking we clearly saw the box, but there is no box when we see the aerial shot as they leave.
When Marlon is restocking the vending machine, he pulls yellow candy bars out of a box to put into the second row, the camera angle changes and when it come back again there was much fewer candy bars in the second row, not to mention they are no longer yellow. They're brown, again the angle changes and when it switches back the bars are again yellow and there are much more of them.
When Christof touches the huge screen with the image of Truman sleeping, his hand casts a shadow. The control room is not sufficiently lit to cast a shadow. The huge screen, acting as a light source, would cancel out the shadow. This reveals that the scene was composited using a blue screen.
The goof item below may give away important plot points.
Incorrectly regarded as goofs
At the start of the final scene (when the boat comes to a halt), the shadows fall to the right. By the end of the scene the shadows fall to the left. This could be from the artificial lighting within the set.