Had a support call come in that someone had lost access to a document library. Turned out that what had happened was that one of my colleagues had been asked to make a folder in the document library 'secure', accessible only to a restricted group of people. What he had done was to edit the permissions for the whole document library.
This request is actually pretty unusual for the sites we're running, We have been asked for 'secure' document libraries before but not to 'secure' a folder within a document library.
Securing a document library is done by selecting "Document Library Settings" from the Settings menu when in the Document Library then Manage Permissions from the settings page that appears.
To do so for a folder first navigate to the parent folder of the folder you want to secure, so if the folder is DocLib\Parent\Child you would navigate to DocLib\Parent. OPenh the Edit menu for the folder you wish to secure and select Manage Permissions. This will open the permissions page for the folder. Unless this folder has previously had it's permissions edited the only available menu will be Actiions. You will need to open the Action menu and select Edit Permissions as it will be inheriting it's permissions from the parent folder. This will cause the New menu to appear and will break the link between the permissions for the parent folder and those for this folder. You can then add and remove users and groups via athe New and Actions menus. If you change you mind you can also choose to return to inheriting the permissions from the parent folder, from the Actions menu.