When you demo a product, you can Goodhart's law yourself.

When you use a product, you stick to the paths that will be most useful.

To demo a thing you intuitively stick to the paths that work, not that are useful.

You Goodhart's law yourself to think it's working by focusing on what demos well.

You'll forget that what matters is not what works but what is useful.

The creator of the tool might forget to look for usefulness and only look for what works.

Someone who didn't build the tool only cares if it's useful, they'll never get confused.