I'm using the latest version of Turbo (beta 4 at the time of writing) and I've noticed that when you hit a 404 page it causes the following error in the console:

Uncaught DOMException: Failed to execute 'define' on 'CustomElementRegistry': the name "turbo-frame" has already been used with this registry

This only happens when hitting a 404 from another link... if you refresh the non existent page the error doesn't happen.