just after they mention at the beginning of the life show the Ecosphere (studio), they are talking about intruders in the show, the first one is the guy jumping out of a Christmas present, and the second is a paratrooper come falling from the air, how could he, as the Ecosphere is a closed studio?!?
He can make it in! In the same scene, there is a man entering the dome. Also, did you ever see the ending. I know there is security but it can't always be watched! Make up a real slip.
This isn't a slip-up, just common sense. If someone wanted to enter from the top of the dome and expect to survive the trip to the ground, they'd need either a parachute or an incredibly long rope. The length of rope required would be too bulky to carry, so parachuting is the only other way.
THIS IS NOT A SLIP UP!!!!!! It was BECAUSE of those incidents that they built the "biosphere"--for security. Watch that explanation scene again where they give the background on Truman's early life, and your path out of the darkness will be revealed by the light of truth.