unable to use profiles in create mode with valid mixed fields

Under certain circumstances, a profile in create mode will prevent access with the following message: Profile search, view and edit are not supported for Profiles which include fields for more than one record type.

...even though the different record types are valid, such as a mix of individual and contact fields.

For example:
http://drupal.demo.civicrm.org/civicrm/profile/create?gid=12&reset=1

That is a profile that ships with Civi and is used in the sample data for events. It contains only first name, last name, and email fields – and should be accessible through profile create, but is not.

If you create a second profile with the same three fields, it will be available in profile create mode. So I believe the issue is that once a profile is used for an event (likely contrib pages as well) it is marked as a mixed record type, and is no longer available as a standalone profile – even if no participant type fields are used in it.