On the left is a new default scene, on the right is the scene I somehow broke. 
Where did things like my Render, Render Layers, Scene, World and Texture tabs in my Properties panel go? :eek:
And what did I do to make them disappear?
Ooh! Thanks. 
Could you quickly explain to me what this pinning feature does and what its purpose is?
Normally the properties panel shows the properties of the current active object so if you change the active object the properties will change. If you pin them then the window will only show the properties of that obejct. You can therefore have multiple properties panels showing the properties of multiple objects at the same time.
The render / render layers / scene / world properties are not associated with the object so you don’t see them when you pin the properties of an object
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I see, thank you very much! Now it all makes sense to me. 