Error: "File '\\servername\backup\undo.bak' is not a valid undo file for database 'database_name' (50786)

This, or similar error displays when attempting to restore any backup file with Standby and undo file:
Error: File '\\servername\backup\undo.bak' is not a valid undo file for database 'database_name', database ID 1.

1. Corrupted undo file (usually caused by copying file to different location)
2. Using the wrong undo file.

NOTE: This is a SQL Server error and not due to LiteSpeed.

WORKAROUND1:
Restart all restores from the latest database backup.
http://www.sqlmonster.com/Uwe/Forum.aspx/sql-server/16653/not-a-valid-undo-file-for-database

WORKAROUND2:
Restore WITH NORECOVERY.
http://sql-server-performance.com/Community/forums/p/4812/31229.aspx

If neither of the above workarounds is successful, please contact Microsoft support.

SQL Server native log shipping may also have the same error, if the undo file (.tuf file) is not correct or missing:
http://social.msdn.microsoft.com/forums/en-US/sqldisasterrecovery/thread/c9bd9b7a-77bc-4663-8d6f-ed5c5ba1e075/