Keeping library books on Kindle for extended times
Here is a blog post about the ability to keep Overdrive library books on the Kindle for extended times.
Seems that if you keep the wireless on your Kindle off library books will not expire.
The good faith way that people might see this happen is if you are using your Kindle in a remote location without access to wireless for an extended period. (e.g. Person in Navy that has Kindle at sea for 6 months)