I’ve just spent 20 minutes staring at group permissions on a server and banging my head of a desk because my user couldn’t write a file in a particular directory.
I was absolutely *sure* the permissions were right.
Directory was on an NFS share. Turns out that there’s a limit to the number of unix groups that work with NFS (it’s 16). If you’re in more groups than that it ignores them.
There’s a detailed discussion of the problem at http://nfsworld.blogspot.co.uk/2005/03/whats-deal-on-16-group-id-limitation.html.