Thats your problem. go into hammer find the offending overlay (may not actually cover that many brushes) and delete it compile it again to make sure it works then try adding it back in smaller or in different location.
Its so nice it even gives you the co-ords of the overlay!
Note: If interlopers comes back negative then READ THE LOG YOURSELF The error usuall falls right out.
Edit: just gotta keep hammering in this compile log checking business, the more we say it, the more people will pick up on it. I stress it every time I say it, not because i'm having a go at you, but because its important. people need to become independant, one of the simplest ways of doing that with mapping is checking out errors yourself, whilst using interlopers isnt strictly independant its damn close to it, then if your best friend for finding errors fails read the log yourself, it only takes a moment.
