The scenario is that all the required fields are before the optional fields and the user intends to fill out some of the optional fields, an accidental enter key will deprive the user of entering the intended optional field data.

If the required fields are after the optional fields and user fills the required fields out first then proceeds to think through the optional fields above and accidentally hits enter, same story.

Currently, a Captcha or a required checkbox "Ready to Submit" next to the Submit button is the work around to prevent accidental Submission. 

Because this Enter Key behavior is standard, an option to modify the Enter Key behavior on the form may violate accessibility standards. 

What is needed is some confirmation on the Enter Key submit to prevent premature submission and ensure the users intent is not aborted.