I just posted a book to someone (WL multiple thing) and I went to her list, it said you are posting to X member and then it went to Y. I feel bad cancelling it ..I had so many cancelled on me because of this and I know it's a bit disappointing. 

Since I promised this book to someone else, how do I cancel it. I just posted it and I don't see a way to cancel this.

I do not believe you can cancel it. Just has to time out.

The worst part is that I think the person it went to is an abandoned account. So it will have to go to someone else and then I think if that person accepts it, it can be cancelled. I wish they'd get this fixed. Disappointing all around.

Since it glitched into FIFO, you'll only be able to cancel it once it's been accepted. If it's WL and it times out then it will bounce to the next in line unless you put your account on vacation hold (if you do it individually or just for your Bookshelf it would make you lose your place for your Bookshelf titles) and remove it after it times out/cancel and remove once it's accepted.

When this happened recently to me, I sent a PM to the member to whom the book was accidentally offered, explaining that the situation was due to an ongoing malfunction of the website.  I asked her to accept the offer then I clicked that I couldn't mail the book.  That kept her place in the WL and the book wasn't offered to any other members.  The member was thankfully very understanding so it all worked out well in less than a day.

Reach out to PBS by the Contact Us link and ask them to cancel it. They will and then you can buddy it.