I recently restored a backup from a production server to a test server. Everything went fine, but one login seems to be giving some of my applications trouble. After digging around a bit, it seems this is common, but im not sure why. After reading a few blog posts, it seems the solution would be to re-link the login and the user with sp_change_users_login?
So I tried it, and it works. Sure enough though...before running that, I looked at the user mapping for the login, and the DB that it was failing to login to, had been "un-mapped". Ive seen this happen before with restoring DB's but like ... why does this happen?