On my Propel Foundation instance, I had implemented Salesforce for Volunteers. (The Trailhead is very well organized & thoughtful!)

And when I was getting all excited to confirm proof of concept by using the Force.com site to register for Volunteer Shifts, I got a series of errors that didn’t make much sense at first.

Click on the image to launch the Sway that explains my findings.

Volunteers for Salesforce Force.com Site
A view of the Sign-Up portion of the Volunteers for Salesforce Force.com Site