I got a PM from the receiver of a book that I mailed. and marked mailed, yesterday.  They wanted me not to mail the book because they had not requested it, and had no idea of how it got ordered.  I suggested that they contact TPTB.

Has anyone run into this problem before?

My guess is they had it on WL on autorequest and forgot about it.

If it wasn't  WL book then they could have ordered it by mistake.  It's not your fault though.  They need to take it up with PBS. On the off chance they don't mark it received-contact PBS. 

I had someone complain because I sent them a hardcover book when they wanted an audiobook. She was convinced that she had ordered and audiobook.  The listng said hardback and she only paid 1 credit.  But she was convinced that 'PBS" ordered the wrong book for her.

Looking at the WL/bookshelf of the requester, I think they got a title confused and didn't look at the book data before ordering.  This book has tons of copies in the system & isn't of the genre on the WL & Bookshelf but has a related title.

You bet I'll be emailing TPTB if this one goes lost.