Have you ever bought a book at a secondhand bookstore and found something unexpected left inside that the previous owner used as bookmark?
If you have, you are not alone. Unusual bookmarks left in used books seems to be a global phenomenon, and a blog post  written by someone with the Naver Matomer username of “Outstanding Individual Talent In 28 Seconds” (28秒に一人の逸材さん ) catalogs what some of these bookmarks look like in Japan.
Many of the bookmarks, before being shared on Twitter as photographs, were found in titles purchased at Book Off , a nationwide chain of used bookstores in Japan.
ブックオフでようやく巡り会えた乙一の｢夏と花火と私の死体｣、帰りの電車で読み終わった。この頃から淡々と恐ろしい事書いてるしオチも相変わらずうわぁ…ってなるキツさだったんだけど、一番怖かったのは最後のページに挟まってた落書きだった。 pic.twitter.com/zhSra89C0l 
— 水玉星人伊藤 (@meijimilchi) July 5, 2014 
I finished reading Otsuichi's book “Summer, Fireworks, and My Dead Body” , a book I finally found in Book Off, on the train ride home. The author described some very graphic things indifferently, and the ending made me gape out of goriness, but the scariest thing was the doodle that was between the last pages.
Other bookmarks showcased in the Naver Matome blog post are cryptic and sometimes unsettling.
中古本に謎の記号が書かれた紙が挟まってた…。まじめに怖い。 pic.twitter.com/xbx9bcUGxD 
— はるひと (@haru_hito) June 23, 2014 
There was a paper in my second-hand book with a mysterious code on it… I'm seriously scared.
古本買ったら挟まってた 狂気を感じる pic.twitter.com/mxF9YjqV6r 
— niՏhi Ꮶ (@cyberp072) October 3, 2014 
(The doodle features cryptic, nonsensical Japanese)
Tweet: I bought a book and can feel the madness contained within.
Some are more whimsical:
古本にブロマイド挟まってた… pic.twitter.com/ac7agWdKzp 
— нацко надя ёда (@natsukoyoda) January 26, 2015 
There was a small portrait picture in my second-hand book…
While others are very creative:
古本に挟まってた手作り(スーパーのチラシを切り抜いた)しおりがかっこいい。 pic.twitter.com/AiAcOotz7q 
— 小銭 (@_kozeni) March 4, 2015 
This handmade (a cutout of a supermarket ad) bookmark in my second-hand book is so cool.
There are bookmarks that seem to somehow match the book they were found in:
古本って良いね！！ pic.twitter.com/83OyDSqahU 
— fudao (@kou294) May 9, 2014 
Holy crap, this copy of Dogra Magra  I bought at Book Off has something in it. There's a Hello Kitty (Sanrio ) bookmark. And on the bookmark are hard-to-read characters with little pronunciation guides written next to them. As someone who reads the (surrealist detective novels) of Yumeno Kyusaku , I cannot hide my excitement! Used books are awesome!
Sometimes there are unexpected rewards to be found at Book Off:
ブックオフで買ってきた108円均一本に4千円挟まってた。 しかし、なぜに新札と旧札？ まぁ 得した。 pic.twitter.com/azOefbWK1S 
— DSW (@Delusion_SW) January 25, 2015 
So I bought a book at Book Off for 108 yen (about US$1) and there was 4,000 yen (about US$40) inside it. But why was it both the old and the new 1,000-yen bills? Whatever, I came out ahead.
Or even hints at the life of the previous owner of the book.
1995年3月19日に撮られた写真。 pic.twitter.com/XwgOjhw3k6 
— まーさ(24)→カノンChildren (@tchitchiman) November 10, 2013 
This was in some manga I bought at Book Off. The photos were taken on March 19, 1995.
More odd bookmarks can be found in the Naver Matome blog post .