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.