This is the second time the same exact thing has happened.  Someone went to my bookshelf and ordered a book from it, but the book ended up being requested from someone else's shelf. 

In both of these instances, the only reason I found out about it was because the person was also requesting a freebie from a deal I was offering.  So the next day, when I still didn't see their request, I PM'd them about it and they both said somehow the book had been ordered from someone else's shelf even though they ordered it directly from my bookshelf.

Anyone have any idea how this could happen?

The member ordering has to follow very specific steps to order directly from your shelf.  I'd forward these instructions to them (from the help center):

• Go to the member's bookshelf
  • Locate the book on her bookshelf.
  • To get to a member's bookshelf:
    • from a forum Post or a PBS Profile: click the button
    • from a Personal Message: click her highlighted name in the To:field.  This will take you to her profile (if she has one)  or bookshelf
• Click the Order This Book ON HER BOOKSHELF (and nowhere else) in order to be sure the request goes to that member.
  • Tip: Before you click the button, look at the page you are on. If "[member]'s bookshelf" does NOT appear at the top of this page, you are ordering from the Book Details page, and your request may go to someone else.
  • If you have clicked the book cover/title on the bookshelf to read more about the book:
    • you will be on the Book Details page (which also has an "Order this Book" button on it). GO BACK to the member's bookshelf to click the "Order this Book" button there--use your browser's "back" button.
  • You can confirm from whom you are ordering DURING the process of requesting the book:
    • by clicking "Order More Books from this member" and confirming that you are taken to the intended member's bookshelf.  Simply complete your order from there, if it is the correct bookshelf.  If it is not the correct bookshelf, navigate away from the request process try again.
  • You can confirm from whom you have requested the book AFTER completing the request process:
    • by clicking   on the transaction on your account page, and seeing whose name "pops up" in the To: field of the empty PM. If it is not the intended member's name, cancel this request and try again. Don't send a Personal Message unless it is the correct sender, though--if a requestor sends a PM to a sender during the one hour "waiting period" (see below), the waiting period will be terminated, and the request will be submitted to the sender immediately.
    • You can cancel at any point up until the wrapper is printed by the sender. Once that has happened, the requestor can no longer cancel, and should not ask the sender to cancel either.

Lisa, thanks so much for your answer.  I'll bet it was the fact that they ordered from the "Book Details" page.  I would have assumed that ordering it from that link would still have placed the request from within the bookshelf I was browsing.  But apparently, that's not so.  I'm glad to know this now.

Thanks again!

If the other member hasn't printed the label yet, they can cancel the request and then order it from you.  Like the prev poster said: they probably checked the book description and ordered it from there instead of your shelf.

Thanks Mary.  Luckily, in both cases the labels hadn't been printed and both were able to cancel and reorder from me.  But neither of them (nor I) could figure out how it happened when they'd ordered it from my bookshelf.  I believe we have the answer now, thanks to users like Lisa who aren't afraid to look them up in the help section.  LOL!