You know how sometimes green screen shots in movies and TV are, well, obviously green screen? Something just looks off about the lighting or the background and you can just tell? What I'm wondering is, what specifically are they doing wrong that even I, someone who doesn't know anything about photography or special effects, can see the flaws in the shot. What should they be doing with the shot that they aren't (either due to money or time or something else)? Examples would be Indiana Jones and the KotCS (jeep chase, the whole ending), X-Men Origins Wolverine. Some in Chronicles of Narnia: LW&W. Tons of shots in Die Another Day. And I know TV works on even tighter restrictions, but Lost has a lot, in particular the shots inside the helicopter from "There's No Place Like Home."
I got to thinking about this because I just watched the first Mission: Impossible film. I could be wrong, but I assume the entire train finale was done on green screen since I find it hard to believe they were running around on top of a real bullet train, and yet I didn't see anything wrong with the shots, it looked totally real. Maybe people more experienced than me can spot flaws, but I didn't see them, while in more recent, more expensive films with bigger effects budgets, they were obvious. So what's going on?