If I login to the site using the LinkedIn or Twitter modules, I am given a cookie like:

openid_provider: twitter

Then when I logout, the openid_provider cookie remains. However, when I then return to the login form and try to login using a Drupal username/password, I get the error:

Sorry, that is not a valid OpenID. Ensure you have spelled your ID correctly.

The error is because JavaScript code is being submitted to the form, my post data looks like this (copied from Chrome web inspector)

openid_identifier:javascript: window.location = $('a[href$=
name:pcavanaugh
pass:Password text here
form_build_id:form-iCLAzlSbR_0m5eBopECE6UGHZgaJyBBvW_6CVahI-DY
form_id:user_login
openid.return_to:https://www.MYSITE.com/openid/authenticate?destination=user
op:Log in

The openid_identifier javascript code is coming from openid_selector_twitter.js or openid_selector_linkedin.js, whichever is in use.