It was a vague feeling; nothing concrete, nothing he could put a name to, just an uncomfortable sensation scratching low at the base of his brain.

Rule number one: Never take your eyes off the Mist. The Tenth is the only reason it works, and that's because Tsuna doesn't know how to give up on anything. Or anyone. And when it's the Tenth asking, somehow everyone just seems to end up going along with it one way or another in the end.