I got a backup of a database from a client, and I wanted to restore over a newly created database. When I selected the file, the Management Studio went berzerk:

Cannot open backup device ‘C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Backup\backup.bak’. Operating system error 5(error not found).
RESTORE HEADERONLY is terminating abnormally. (Microsoft SQL Server, Error: 3201)

This is a true WTF heart stopper, I thought that all the backups were invalid since we’re running the site

Afterwards it turned out that the client backs up the database files to a networks share, so magically somehow the permissions has been altered for a local restore. According to this hint by Stevied, I added the Network Service user as an owner, and voila: all functionality magically returned…