Continuity mistake: When he hands Tony the autopsy report, Ducky has hair falling over his forehead, then combed back, then over his forehead again repeatedly from shot to shot. (00:07:15)
Other mistake: Season 5, Episode 12, "Stakeout": The picture on McGee's camera phone is just wrong. It shows not just the man in front of him, but it also shows a hand, holding a camera phone - McGee's camera phone. In other words, the phone is showing a picture of itself and the man, instead of just the man. (00:40:25)
Continuity mistake: When they discover that one of the competitors most likely has the radar, Agent Lang doesn't have a page hanging free. Camera cuts and there is one. (00:28:40)
Ssiscool
Revealing mistake: When the body is found there is a pool of blood near the head, with blood running down his head. However, the pool of blood is away from the head and not touching it as would be expected. (00:07:30)
Ssiscool 
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.