Contractors are often surprised by an audit bill months after the policy ends. One of the most common reasons is money paid to subcontractors who didn't have their own insurance, or whose certificates the contractor can't produce.
On many contractor GL and workers compensation policies, payments to uninsured subs are treated as if those subs were the contractor's own payroll or exposure, and rated at the contractor's class. That premium is charged at audit, after the work is done.
The short version
Collect a current certificate from every sub before they start, for the dates they work. No certificate at audit usually means the carrier charges premium on what was paid to that sub.