Or, ask two questions, where (on at least one) it's fairly easy to get an inefficient solution, but requires more thought to get a more efficient one.

You don't necessarily demand perfection on this test though.

One mistake that companies make is that they think they want to set a lower bar, so they make the questions easy. But then, in order to distinguish candidates, they basically require perfection. Now, a simple mistake can "fail" a good candidate.

It's better if they make the questions harder, but just don't require perfection.