I've been reading my pets a lot of books today, and it was all without incident until I got to this book:

[item2=A to Z Plant Encyclopedia]

I bought 36 of them to read to my 36 pets, but when I got to the last one I got the 404 error saying the item didn't exist. I backed up to the inventory page and saw that it said x36 underneath the encyclopedia. (In fact, I checked it more than once to make sure I was looking at the right book.) But when I did an account search it said I only had 35 of this book.

So I didn't accidentally read it to the wrong pet, and I didn't put it in the vault or shop. I also couldn't have donated or deleted it, because I have those buttons removed via CSS. The book just...disappeared.

There was some lag going on while I was reading this book to my pets, but there was lag with other books as well, and this is the only one that vanished.

That's in fact not an item issue, you should have posted it in the Site Functionality Issues part, if at all. Unfortunately 50xs are reknowned to eat items all over the place.

You should file a ticket to get reimbursed.