1. Add a 'file' resource
2. Add more than one file (by uploading for example) - note that if only one file is used then this error does not occur
3. Click 'save and display'
4. Get error message shown in http://screencast.com/t/OWU0YWFhOTg

After messing about for a bit I realised that there is a button to the left of the files (which doesn't show in the theme on this site) which allows me to 'set main file' from the list, but the error handling seems a bit vague and inconsistent with what happens elsewhere in Moodle.