Dear …
RE: __(Name of project) __
We want to thank you for catching our mistake of __(type of mistake) __ that we made on the __ (name of project) __. Our company strives for the highest quality control, yet we will on occasion, experience an oversight created by human error. We are quickly making the appropriate corrections and will contact you just as soon as they are complete.
Please accept our apologies for any inconveniences this may have caused you. We appreciate your patience in this matter and thank you once again, for alerting us to this situation.
Sincerely yours,