This is one more recent experience which I faced.
I upgraded CRM 4.0 Organization (which was originally upgraded from CRM 3.0) to CRM 2011. everything went fine with very minor hurdles. We tested the upgraded organization by login into that organization my domain credentials. We are able to get in into the site and everything works fine.
Once after sending our confirmation email to our client, suddenly our client reported that they are not able to get in into the site even after providing their domain credentials. But earlier they never faced this issue with the CRM 4.0 Organization.
Here is the error they are getting.
404 – File or directory not found.
The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.
After doing little bit R&D first we found that there are two user records in the system with same domain name. May be one was created on CRM 3.0 and deactivated and after upgrade to 4.0 one more user got created with same domain name. (We are not sure how it allowed)
Now we understood that the user mapping is not done properly at the time of CRM 2011 upgrade process. To fix our issue and to allow the user to login into the system we changed the OLD user record to point to other domain user. and we did the same exercise with the current user record also.
Once after that we repointed back the existing user to use the same domain credentials. Now the user mapping is done properly for the user and is able to login into system.
Hope this helps to anyone 🙂