An end-user sent a mail along with SharePoint Document’s URL to another user. When receiver tried accessing the document, it resulted “404 Page not found” to him. However the sender is able to access the document successfully!
When I tried with Farm Admin access, even I got “404 Page not found”, after an hour of going crazy, found the document was checked out by the sender!
Get into the Document Library settings, Manage Checked out files, and then took the ownership of the file, checked it in. Solved the issue.
In another case, this issue came up again. End-user received “The Specified name is already in use” since another file checked out has had the same name!