If you give yourself a book in survival with recipe or in creative with /give command, you can clear the page, but if you save the cleared page, it don't save. So you can again open the book and the old text is shown.

To reproduce

1. Give yourself a writable book
2. Write something in it
3. Click "Done"
4. Open it again
5. Remove everything you've written
6. Click "Done" again
7. Open it again
   → Your changes did not get saved