Dell i7 - 3450M
Windows 7 Enterprise SP1
tested on various other computers
When rendering a scene, the combined pass is rendered even if it is deselected. In this case only the AO pass was required and selected.
If the combined pass is required for the other passes, then it should be removed from the list or clarified by not allowing the user to deselect it.
Open blend file (any, basic scene)
In passes, select AO, deselect Z and combined
- rendering this will render the combined and AO pass
in node editor: compositor, connect render layers:AO (render layers:combined is not on the node) to composite:image
- rendering will render the combined pass, and then display as output the AO pass.