A postal employee in Alabama who complained for approximately two months of coworkers wearing confederate flag t-shirts in the workplace until the Postmaster finally instructed the supervisor to start sending the employees home to change was found by the Equal Employment Opportunity Commission (EEOC) to have stated a viable claim of discriminatory harassment which requires further investigation by the Commission after his complaint was initially dismissed. In reversing this dismissal, the EEOC stated:
While isolated incidents may not create a direct and personal deprivation sufficient to render an individual aggrieved, the Commission has held that, under certain circumstances, a limited number of highly offensive slurs related to a federal employee's race may in fact state a claim or support a finding of discrimination under Title VII. See Brooks v. Department of the Navy, EEOC Request No. 05950484 (June 25, 1996). Moreover, Complainant has alleged that it took the Postmaster nearly two months to finally take action to stop his coworkers from wearing the offensive t-shirts.
Thus, as is important in most cases and opinions, it's a case-by-case analysis and it often will come down to the employer's response to an employee's complaints that either make or break the day. The remainder of the opinion can be found here.