Absolutely Nothing
No warnings. No pairings.
Disclaimer: Um. Yeah. Not mine. Just a sort-of response to the many theories.
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"That gives me the heebie-jeebies and you know it." Harry to Bob, Episode 7, Walls
But the question is . . . why?
We all know that Bob, when he wants to, can feel or sense things with his hand – can activate them with some sort of latent magical ability. He can reach in and "feel around" and see pages in a book when it's closed.
But what about other people? Why do they find it so very disturbing when they walk through Bob, or vice-versa? Two entities occupying the same space . . .
Is there a cold spot? A tingling? Warm and cold sparks at the same time? Some vague, unknown duality of magical essence?
Perhaps its none of the above. Perhaps Harry Dresden gets the heebie-jeebies when occupying the same space with Bob not because of cold or weird tingles. Maybe, it's because when he passes through his friend, who usually looks so solid and real and alive, Harry feels . . . absolutely nothing.