When can an employee’s transfer be considered illegal retaliation?

To bring a case of retaliation for complaining about discrimination or harassment, employees must show that they suffered some sort of “adverse employment action” in response to their complaint. That’s easy if the employee is demoted, fired or transferred to a less desirable position. But what if the worker experiences more subtle retaliation, like having to do more work or being transferred to a potentially better position that doesn’t pan out?