Can you think of other examples of things you might understand theoretically, but not practically?

Best example would be Carnot engine. It is possible in theory not in real life.

Explanation:

In real engines, the heat transfers at a sudden change in temperature whereas in a Carnot engine, the temperature remains constant. In our day to day lives, reversible processes can't be carried out and there is no such engine with 100 % efficiency. Thus, the Carnot cycle is practically not possible.