[Anthony G Williams]

The two examples are rather different. In theory, a bunch of monkeys could type all of Shakespeare - except that the statistical probability is so vanishingly remote that the process would probably take many times longer than the universe has existed or is likely to exist (no doubt a statistician could work it out). So it's an example of the theoretically possible being practically impossible.

A cubical planet, on the other hand, would conflict with the laws of science, and would thus be theoretically as well as practically impossible in this universe. You would need to postulate a universe with very different scientific laws to make that work.