Make sure you train employees how to lodge work-related complaints with the company. The benefit: A clear complaint procedure provides a defense if employees lodge informal complaints outside of your process. As this case shows, an employee's casual complaint to a co-worker could be considered "protected activity" that serves as the basis for a retaliation lawsuit.
Recent case: Donna Hazen complained to her co-worker about gender bias in the company. Soon after the co-worker relayed Hazen's comment to, Hazen was transferred to a new position. Hazen sued, claiming her transfer to a less-desirable job was in retaliation for her complaint. While a lower court sided with the company, a federal appeals court (in an unpublished decision), said that Hazen's conversation with a co-worker regarding unfair treatment was a protected activity for purposes of bringing a retaliation case. (Hazen v. Modern Food Services Inc., No. 03-4014, 3rd Cir., 2004)