Jonathan Young and Dana Hefter wrote a great article in the August 2014 ABI Journal examining In re Rodriguez, Adv. No. 11-07012 (S.D. Tex. June 5, 2013), in which Judge Isgur held that an attorney signing a proof of claim on behalf of a creditor became a fact witness, and consequently waived attorney-client privilege and work product immunity through offensive use. Their article is a must-read for all bankruptcy attorneys, and any non-bankruptcy attorneys who find themselves confronting Official Form B10 on behalf of clients.
I do not have much of substance to add to Jonathan and Dana's article, other to thank them for being thorough and for having excellent timing. I read the article less than a week before I was getting ready to prepare a stack of proofs of claim. I'm sure I've signed claims for clients before out of convenience, but thanks to this decision, I will never do so again. I don't want to end up on the business end of a deposition table.
You can check out the article at ABI here -- The Involuntary Witness: Privilege and Waiver Issues Raised by Rodriguez (paywall warning, but you should join the ABI anyway). A copy of the decision is available here.