Kubuntu Linux 14.04
nVidia GT430: VGA compatible controller: NVIDIA Corporation GF108 [GeForce GT 430] (rev a1)
nvidia driver 340.65
Broken: 2.73a , 2.73.9 (2.74 testbuild)
Short description of error
Is not possible to copy nodes from one instance to another instance. Blender uses the clipboard only if you copy/paste into the same instance of Blender. If you copy in an Blender instance and then paste into another Blender instance, then this second instance reports that the clipboard is empty or clipboard nodes are an incompatible type depending of the clipboard contents.
Exact steps for others to reproduce the error
Open one .blend project in one Blender instance. Open other .blend in another Blender instance. Select nodes o node groups (Shader or Compositing) in the first instance and press Ctrl+C to copy them to clipboard. In the second Blender instance put the mouse pointer over the node editor and press Ctrl+V to paste copied nodes from clipboard. Blender report "Error - Clipboard is empty" or "Cipboard nodes are an incompatible type" depending of the clipboard contents.