When trying to set up Facebook Connect for our Android Forum site, we got a nasty error after people completed authorising the site:

[#1090091] An error occurred while trying to authorize this Facebook connection

There are plenty of people who write about this problem if you Google for it, but not many solutions.  Luckily IPB support is superb and solved our problem for us.  So if you are getting the "[#1090091] An error occurred while trying to authorize this Facebook connection", go to your Facebook App Advanced settings, and under Migrations disable everything except for Remove Deprecated APIs.

If that doesn't work try IPB Support, they are really on the ball.