When a site contains two forms (e.g. sign-up and login) only the first form is filled in. That however is the wrong form in about 50% of the cases.
An example you can find here: http://www.pronunciationworkshop.com/account/login
This page has the additional complication, that the password field has the same name attribute in both forms.
I can think of two possible solutions:
1) Require the cursor to be in one of the forms on pages with multiple forms. Then fill in (and potentially submit) only that form.
2) Fill in all forms and disable auto submit, if the page contains more than one form.
Both solutions could be restricted to only pages, that contain more than one form with a password field.
As it currently stands, 1Password is unusable on those pages.
1Password Version: 6.0.2
Extension Version: 18.104.22.168 (Chrome)
OS Version: OS X 10.11.3
Sync Type: Not Provided