Comment 5 for bug 1360244

I stumbled upon a similar issue, and I'm using a devstack master.

There is a much easier way to reproduce that the steps indicated by the OP:

1. Click on "Launch Stack"
2. Set Template Source to "File", and select a template file. Do not click the Next button.
3. With a filename entered, switch the Template Source to URL, and enter a valid URL to a template.
4. Click Next. The error "Please specify a template using only one source method." is displayed.

The problem is that the form fields that hold File, URL and Direct Input templates are different, and there is always one visible at a time. The form validation expects only one to have content, but since two of these are always hidden I think validating that the hidden fields are empty is pointless, the form should take the template from the correct field, and ignore any content that exists on the other two.