I am definitely the type of person to use any thing around me as a bookmark. A scrap of paper, a pen, my phone, another book, a book-jacket flap, a receipt, a dollar--anything. I'll even use the poor man's bookmark: remembering my page number before closing the book.
Yes I try to be responsible and keep my Ten Dollar Books bookmark in the book somewhere while I'm reading, but sometimes I'm reading more than one book at a time, so I need back-up bookmarks, on-the-spot bookmarks. Something just to grab real quick and get back to later.
It's to the extent that I don't even remember what I used as a bookmark in certain books. And I might not return to a book for a while. Like a couple weeks ago I remembered randomly that I had a parking ticket to pay. Couldn't find it and couldn't pay it without the info on it.
I finally just got back to working on this conference paper I'm writing, and the ticket was wedged inside my grad school copy of Confessions of an Opium Eater. Yikes.