Some very long calculation that a human would need time to parse, let alone compute, while instead a bot can do in milliseconds
I'm convinced that the bots will soon do better than humans on image recognition.
With speech recognition, humans have shown worse performance with completely random words and letters, and this was about at least a decade ago when the tech was worse. For example, a bot would likely identify DBDBBDD in speech better than a human.
Add a honeypot field for humans (visible) and bots (hidden). If visible honeypot field is filled or hidden one not filled, that means you have a human
How about a form that required clicking on an invisible button? A bot would see the button and click on it, while a human would just read whatever else was on the page and close the page.