Covering All Bases
If you take outside quotes as an aid to estimating Special Construction, review the scope of work covered in those quotes. It may be that the outside agent has wrongfully assumed that you will provide traditionally supplied items such as excavation or an unloading crane. Check to make sure that it is covered somewhere in your estimate, and that it is not carried by both parties.

Interfaces
Review all documentation to ensure that all interfaces, such as electrical connections and control wiring, are accounted for. These items have a habit of falling through the cracks of an estimate.