Sometimes, a project manager must deal with a disgruntled client. They may be unhappy with one thing or many things, but if their feelings are strong enough to make a complaint then that complaint needs to be properly and fairly dealt with.
Unfortunately for project managers, complaints are not a rarity in the world of project management. It can seem very unfair to get criticism on a project when in the majority of cases a lot of hard work has gone into getting it successfully delivered.
However