Continuity mistake: In the alleyway as they go to enter the building, the team is by the first car. Camera cuts and now they have passed the second car and are halfway up the alley. (00:20:20)
Continuity mistake: When Tony is talking to McGee at the start, Tony has his hands in front of him or to his side depending on the shot. (00:05:30)
Continuity mistake: Fornell gives Gibbs a juice box when Gibbs stays with Fornell. Gibbs puts the straw into the box with the straw closest to him. Camera changes and now the straw is at the other end of the box. Since there is only 1 hole for a straw on a juice box it is clear the box has turned around. (00:25:35)
Continuity mistake: McGee asks Ziva if the FBI managed to contact all the witnesses. When he does he has his ears uncovered. Camera changes and he's got his hand on his one ear. (00:16:40)
Continuity mistake: At the start when Tony is lecturing McGee on bleaching his teeth, McGee has his hands on his desk. Camera changes and one is now on his chin. (00:05:50)
Continuity mistake: When Fornell and Gibbs are talking in Gibbs' basement, Gibbs places a red storage box on some wood and there is a fair gap between the red box and the cardboard box that is next to it. When the camera changes, the boxes are almost touching with no-one touching it. (00:04:15)
Answer: I dont believe it is ever mentioned, but like most pilot episodes the characters are often changed, replaced or even removed in this case.