Major Mistake: Any Explanations?

Rating: 7/10 (16 July 2010)

When the van driven by Yusuf goes off the bridge in the first level of shared dreaming, Arthur should have been kicked. Arthur was only in the second level, and the change from gravity to weightlessness should have kicked him back to the first. Everyone else was in the third stage or lower, so they would not have been affected. Cobb and everyone lower than the second stage would have been trapped there forever since no one was available in the second stage to kick them out of the third or lower. This seems to prove that the final scene was, in fact, a dream. But the blatant disregard of Nolan on this topic makes me suspect that it was merely a mistake. Any explanations?