Today I entered 8 ISBN numbers and the little box turned green (correct) and I clicked "Continue" -- and the system told me I had entered FIVE books.

Since I had written down the ISBNs and brought the list to work with me, I had to do a search on the other 3 numbers (which were all correct).

Any idea why some immediately went on my shelf and others didn't, when the boxes were all green for OK?

Annoying, but not infuriating.

If 3 of those ISBNs were valid but already on your bookshelf, this would happen... that's all I can think of.

Nope, they weren't.

And the two or three times in the past I've accidentally tried to repost a book a second time, I've gotten an on-screen notice telling me only one copy at a time could be posted on my shelf.

You might try posting the ISBN's that didn't go using the add single book option.  I have had quite a few that won't add for whatever reason using bulk, but when you enter them individually they work just fine...it's a mystery to me!

I had this happen recently also.

Once I had entered the ISBN for a Canadian book, which didn't match the USA ISBN.

Once I had tried to post a book not in the PBS master index, had to enter all the info as I would for a book with no ISBN.

Allin all, the system is good but I don't expect it to be perfect!

You may notice on the tab called "my lists" ; there is a tab called "books to repost". If I forget that the book I am trying to post originally came from Paperback swap, and I enter it as a book to post using the usual page for new books, my mistake is caught and the book gets removed from my "books to repost" list.  I think that is pretty cool!

That happens to me almost everytime I post multiple books - 1 or 2 don't post, then I have to go back and do those individually...

There are a bunch of books, in my experience, that refuse utterly to be uploaded through the 'multiple books' option, but will happily post individually.

Maybe if the book has never been posted to PBS before, the database has to go look it up on the internet -- and maybe it can't do that from the multiples page?

Eloise -- interesting idea -- I'll have to look to see if there are other copies on PBS of  the 3 I had to post twice.

I've also had this happen. Weirdly, some books just won't go on the multiple posting. Even tho the box turns green.

Eloise may have nailed it -- one of my books (Eva, Evita -- a really interesting bio of Eva Peron) is the only one in the system.  So maybe if the book is a "first-timer" it doesn't post correctly.