There is a really interesting question which I posed to the NASA near earth team at one of their talks at Ames which was "what sort of evidence would an intra-atmospheric near miss produce?" Which is to say a comet that missed by 50,000 to 125,000' rather than further out. The speculation is that it would result in wide spread fires (the compressive heating of the air in front of it would ignite things). Basically it could transfer a lot of energy to the atmosphere. But most felt that a comet that close would be in pieces and at least some of it would leave a crater trail.