If you can find a book with a sewn binding (or better a pair, as-yet unopened), if you open one per the instructions, then open the other haphazardly as one reads it, you will find that over time, the book which was not opened with care will develop a directionality to its binding causing it to be cocked, while the other, so long as it is read carefully will remain square.

Typical glued texts are more prone to this, but depending on the state of the glue, may go back so square if pressed when square to correct.