Michael Blackburn's answer helped me resolve my issue, but I want to give more detail on my fix.
I have a php app that posts to a user's FB page.
I own two domains:
- http://app.my-web-app.com
- http://app.mywebapp.com (no hyphen)
I built my site off the first domain because it read better IMHO (at least it did at the time).
Some users typoed the url so I bought the second one with no dashes for that reason.
So, one of my users was having the "Given URL" error.
Turns out he was going to http://app.mywebapp.com and the rest of them were going to http://app.my-web-app.com
I fixed everyone by adding all possible redirect URIs:
Granted, there are 100 better ways to implement this, but here is the workaround for now.