What is a common mistake most employees including managers make when handling guest complaints?

The most common mistake that most employees make when handling client complaints is not listening to the full ability.

There are2 types of listening, active and selective. Most of the time in client relations we are selective hearing. We are trying to hear the "why" of the complaint. (What did we do wrong on our part? Is it their fault or ours)

Being an active listener you are able to recognize the clients hand gestures and body posture, you can hear the5 W's and react on the situation as a whole.