when a hopper full of items that isn't locked is placed bellow a chiseled bookshelf, it will constantly set the last interacted slot to the last non-empty slot in the chiseled bookshelf.  It will also give constant updates that an observer can detect.
to reproduce:

1. place down a hopper and fill every slot with 1 of any stackable item which isn't a book
2. place a chiseled bookshelf on top of it and fill it with books
3. place a comparator taking output from the chiseled bookshelf and measure the signal strength
4. remove/add books to the 1-5 slots and note that the signal strength does not change
5. place an observer pointing into the chiseled bookshelf and note that it is constantly firing.