This is to inform you about the rude treatment I received while patronizing <bank
name> while I was conducting business on <date>. The <name of employee or
description of the person's position> <description>. While I am not usually one
to complain, I found this behavior exceedingly unprofessional and would like to inform you
about it so that the problem can be rectified at the highest level.
I have been a customer at your bank since <year> and have always been satisfied with
the service I have received there. I am hopeful that this incident was a "fluke"
and will not be repeated again.
Thank you for your kind consideration of the matter.