The page for pointing Bridge to a Mastodon server's auth flow asks users to provide:

Your full Mastodon identifier (username@domain)

This is a little weird as we have been trained to always type @username@domain. I understand that it is technically user error to put your @ in front of the username but I did that like 5 times before realizing what I was doing wrong. And the error message you get when you do is a generic "something went wrong".

I would ask that in the case of a user typing "@username@domain" either the prepended @ gets silently cut, or that the error message be more verbose, like "Please enter your Mastodon identifier in the form 'username@domain'"