You can't make all of the people happy all of the time. Knowing that, there will be times the performance of either you, your office, or your staff will fall short of expectations. How should you handle it?

If the complaint is even slightly valid, fall on your sword. Take the blame for the issue fully and on behalf of everyone who may have led to the letdown. There is a difference between fault and responsibility... the responsibility is always yours, if not the fault.

There are varying degrees of error. If the mistake or instance was of little consequence assure the internal customer a lesson has been learned and won't be forgotten. If the problem is bigger move on to your problem resolution processes. In all cases assure the person bringing up the complaint that it is being reviewed and thought of. Follow up with some type of resolution or explaination in the future.

If you are faced with a constant low-level back chatter that  "purchasing always makes mistakes" but nobody can ever seem to give specific issues or instances please let it be known to everyone involved that you have an open-door policy and welcome any/all constructive criticism. Never argue, just listen.

Nobody is perfect. Avoid being defensive and always look to be as helpful as possible. When you are faced with criticisms that are unfair or inaccurate just remember that there is a reason the blame is heading your way. Find out what that reason is and improve or change it. Your reputation will go a long way!