I know those letters can be so upsetting, especially if this is the
first on in 34 years! That's a pretty remarkable record on your part.
And sometimes it seems that there are a handful of customers that no
matter what you do, there is no pleasing them.
Without knowing your company or your company's policies about such
'warning' letters, I can tell you that it is usually the 'norm' to
have an employee sign to acknowledge receipt of the letter. This is
usually placed in the employee's personnel file, and may remain
thereforever or may be removed after a certain period of time.
Companies use this type of procedure to document that you have been
made aware of the incident. It protects you, too. Acknowledging that
you received it, distasteful as it is, keeps anyone from placing
documents you've never seen in your personnel file, too.
Had you refused to sign it, your supervisor could note on the copy
that you refused to sign it but he did show it to you and gave you a
copy. The letter with his (or her) notation would be placed in your
file. A refusal to sign would accomplish nothing and make you look
stubborn or uncooperative.
Likewise, although you could have written "signed without recourse",
it would do nothing with regard to what you acknowledged with your
signature (that you got a copy), and might have left a negative
impression with regard to your cooperation. All you did here was
acknowledge that the matter had been brought to your attention.
If you feel the customer's complaints are unjustified, you might wish
to address the points and ask that your letter be placed with the
other letter to present your side of the story. It would seem patently
unfair not to allow you to do so. Just remember to address the matter
factually and not emotionally (this is obviously not the moment to
refer to the customer as a "total jerk").
Consider this, too: It could be that no one agrees with the customer,
but it could be company policy that this is how these matters are
handled. If that is the case, it could also be that no one agrees with
it, it is just something they have to do as a matter of procedure
because of the circumstances.
I answered your question more from years of experience, than from
researching the matter. Hopefully a practical look from someone not so
close to the situation might help put it in a different perspective.
Good luck, and I still think 34 years with no previous complaints is a