Payroll tax liability
Generally speaking you will need to register for, and then file and pay unemployment tax in each state where an employee resides. Some states require payroll registration if any work is being done in that state, even if no employees live there.
State corporate income tax varies as well. In some states a business must file and pay as soon as it started conducting activity within the state, in others the activity must rise to the level of ???regularly transacting business??? before the requirement kicks in. It is important to investigate the state laws before operating or hiring an employee in a state.