I have a directory structure owned by a certain group, and I am a member of the group that owns these directories. I am able to create files in one directory, but not in another, even though the permissions are the same.

alan@bricky:/mnt/storage/media$ stat Music Music\ \(Lossy\)/
  File: `Music'
  Size: 34          Blocks: 8          IO Block: 4096   directory
Device: fb00h/64256d    Inode: 4215424     Links: 3
Access: (2775/drwxrwsr-x)  Uid: ( 1001/   media)   Gid: ( 1001/   media)
Access: 2011-08-19 11:45:03.182586898 -0700
Modify: 2011-08-19 11:44:01.412840027 -0700
Change: 2011-08-19 11:45:02.734603240 -0700
  File: `Music (Lossy)/'
  Size: 6           Blocks: 8          IO Block: 4096   directory
Device: fb00h/64256d    Inode: 1512056832  Links: 2
Access: (2775/drwxrwsr-x)  Uid: ( 1001/   media)   Gid: ( 1001/   media)
Access: 2011-08-19 11:45:03.190586606 -0700
Modify: 2011-08-19 10:34:46.526530313 -0700
Change: 2011-08-19 11:45:02.738603094 -0700
alan@bricky:/mnt/storage/media$ touch Music/test
alan@bricky:/mnt/storage/media$ touch Music\ \(Lossy\)/test
touch: cannot touch `Music (Lossy)/test': Permission denied