A contractor's general liability policy is built for injuries to the public: a customer, a passerby, a neighbor. Injuries to workers are a different story. The contractor's own employees belong on workers compensation, and many contractor GL policies, especially in surplus lines, also exclude injuries to the employees of subcontractors and other contractors on the job.
That second exclusion is what catches people. An injured worker usually can't sue their own employer because workers compensation is their remedy, so they sue someone else on the site, often the general contractor or the property owner. That's an "action over" claim, and it's one of the most expensive claims a contractor faces.
The short version
Read the employee-injury wording on every contractor GL quote. A policy that excludes injury to any worker on the job, not just the insured's own employees, leaves the contractor without coverage for action over claims, which are often the claims that hurt most.