I was working on a WordPress project when I face this problem. It’s not any big deal. I have a form in custom template with 5 fields. But when I clicked on the submit button it shown 404 errors. So then I found that if you are using $_POST, $_GET or $_REQUEST with a post or get method in form the first check form fields name.
Eg. You have 2 fields with a username and email. These names used by WordPress. So just change the name and id of fields and then try to submit the form. It’s just simple.