If the webform is defined so that a page break is the last component, then the previous page has a Next Page > button. If you click it, nothing happens.
I consider this a webform configuration error, but the behavior is not very nice. I think it should display a page with nothing but a submit (and save as draft, if needed) button. If the Next Page > button submits, then what do you if they intended to save as a draft?
Yuck.
Other issues to consider:
#1781490: Conditional fields can cause form to submit unexpectedly
#2204515: Webform preview "back" button submits form
Comments
Comment #1
quicksketch> I think it should display a page with nothing but a submit (and save as draft, if needed) button
I think this would be a good solution, but unfortunately we already consider a page with no components displayed (as would happen with a page full of conditionally hidden components) to mean "skip this page". So with this existing rule in place, I'd expect this would result in the form being submitted. If making a page with no components turns into a mess and just submitting the form is easier, let's start with that fix.
Comment #2
quicksketchOddly after committing #2204515: Webform preview "back" button submits form, this problem is "fixed" when using a preview page (showing a blank page with no components). If there is no preview page, the form still cannot be submitted.
Comment #3
danchadwick commentedFixed by virtue of fixing:
#1781490: Conditional fields can cause form to submit unexpectedly