We have a site that we migrated from our dev box to our test box. Since the
migration, and item such as documents or users, that should show up as being
modified by the system account are instead showing up as being modified by a
user who was removed from active directory many months ago.

If we are logged onto the site as the system account and click on "Welcome
System Account > My Settings" we get an error like:

User cannot be found. at Microsoft.SharePoint.SPList.GetItemById(Int32 id,
String strRootFolder, Boolean cacheRowsetAndId)
at Microsoft.SharePoint.SPContext.get_Item()
Troubleshoot issues with Windows SharePoint Services

If we go to an item that shows as being modified by the former user, and click
his name, we are taken to the same error page as if we try to display the system
account's "My Settings"

If we browse to Site Settings > People and Groups and click the ID that is the
system account, it displays fine. In fact all the users' "user information"
pages display fine.

We have tried restoring this site over and to different sites, and the issue
persists. Other sites in the same web application do not have the same issue.
Can anybody give me advice? I am really stumped.