According to the Fair Labor Standards Act, in most cases an employer can force employees to work overtime. Laws can vary depending on which state an employee works in, but the laws regarding wage orders are required to be posted in a prominent place at work where employees can see.
In the state of California, an employer can force workers to work overtime even if an employee was not scheduled to work at all. An employee may also be called back into work even after returning home for the day or after working 8 hours. The only laws in regard to both situation is that an employee has to be paid for all hours worked.