The Gemara in Sanhedrin 71a tells us a few mitzvot in the Torah that will never happen:
First, a ben sorer u'moreh
בן סורר ומורה לא היה ולא עתיד להיות ולמה נכתב דרוש וקבל שכר
There has never been a stubborn and rebellious son and there will never be one in the future, as it is impossible to fulfill all the requirements that must be met in order to apply this halakha. And why, then, was the passage relating to a stubborn and rebellious son written in the Torah? So that you may expound upon new understandings of the Torah and receive reward for your learning, this being an aspect of the Torah that has only theoretical value.
The Gemara then adds that an idolatrous city will never happen:
עיר הנדחת לא היתה ולא עתידה להיות
There has never been an idolatrous city and there will never be one in the future
And finally the Gemara states that a house that was afflicted with leprosy will never happen:
בית המנוגע לא היה ולא עתיד להיות
There has never been a house afflicted with leprosy of the house and there will never be one in the future
I have a couple of questions:
- Why would Hashem gives us mitzvot that would never happen?
- To expound and receive a reward? Are we experts enough in all other areas of the Torah that we don't need to spend time on what we already have?
- Why specifically these 3? Why aren't there more mitzvot mentioned that will likely never happen?