Objects non-selectable for no apparent reason

This object (which was a family type hosted onto floor slab representing a pertrol (gas) station forecourt mysteriously became non-selectable 

in a worksharing project with a couple of people working on it.

Led to the discovery of a little known function in the bottom RH tray Select Pinned Objects.

This globaly makes any pinned objects un-selectable.




How many hundreds of Revit users have spent hours trying to solve this mysterious happening and blaming it on file corruption or whatever!