ownCloud is a very nice tool for syncing and sharing files and folders in a workgroup or even company, basically it’s an open source replacement for Dropbox or Google Drive that runs on your own infrastructure.
A problem that may occur is when user Alice shares a folder „SHARED“ to a group „everybody“ (all the other users are members of this group).
Then user Bob clicks on the Recycle bin icon to „Unshare“ this folder for him/herself (he opts-out of this group-share even if he continues to be member of group „everybody“). Bob does not see this folder anymore but all the others do. Bob has no way of ever reverting this action and cannot opt-in again.