The easiest one to violate is accidentally turning your contractor into an employee, and if that happens, may God have mercy on your soul, because the local tax agency will not.
It's going to be awfully hard for any taxing authority (at least in the US) to characterize a contractor hired through a service (in the manner described in the article)as an employee:
1. The contractor presumably has other clients/customers
2. The contractor is not in your facility
3. The contractor is using his/her own tools and equipment
There are a number of other tests, but this is actually probably one of the "safer" ways to hire contractors.
Keep in mind that the contractor/employee dilemma exists whether your worker is offshore or local. It may actually be better if the employee/contractor is offshore (from a US perspective), because you would not be liable for most of the taxes/deductions required of an American employee.
The easiest one to violate is accidentally turning your contractor into an employee, and if that happens, may God have mercy on your soul, because the local tax agency will not.