amp-form submission breaks in several browsers.

How do we reproduce the issue?

Problem noticed on a private website, nevertheless it also exists on other amp-form powered websites, most prominently in the examples provided by the Project itself:
https://ampbyexample.com/components/amp-form/preview/

1. Submit the form in the bottom of the page.
2. Observe an error being thrown in the console:

InvalidStateError
v0.js (314,433)

What browsers are affected?

At least Edge 13, 14, IE 11, Safari 10.
Latest Chrome, Firefox, Opera are not affected.

Which AMP version is affected?

Never tested before, can't say if it's a new issue.
Version 1484769855829