The file is currently checked out or locked for editing by another user

One time I got above error while renaming folder in SharePoint 2007 document library, error is because someone checked out the file and at that time I don’t have a way to figure out who checked out the file, so I got different alternate solution to rename folder name inside document library – SharePoint Designer :)

http://hristopavlov.wordpress.com/2008/12/02/the-file-is-currently-checked-out-or-locked-for-editing-by-another-user/