I wanted to share a permissions bug workaround on a mounted image -- just because it's a counter-intuitive technique and it could have other applications.
I wanted to share a mounted encrypted disk image between two users. Whatever I did, however, only the first user to mount the image could create new documents on it. Both users could edit existing documents and both users could create folders (but not files). If the second user tried to create a document they got this error message:
The operation can't be completed because you don't have permission to access some of the items.
Since both users had permissions on both the image file and the mounted image/drive - this error message was clearly wrong. It should be interpreted as "OS X has a permission problem of some kind but I don't know what it is. Good luck".
The fix was to disable 'ignore permissions' on the mounted drive/image. (Default setting is to 'ignore permissons').
With permissions enabled both users can create files.