--- Operating System, Graphics card ---
Win7 64bit, Nvidia Geforce 570

--- Blender version with error, and version that worked ---
Blender r55398 (latest buildbot atm) - no comparable with older (new node merged to trunk)

--- Short description of error ---
Blender crashes if creating a node group, selecting a singular node and then undoing.

--- Steps for others to reproduce the error (preferably based on attached .blend file) ---

In Node editor --> Compositor (not materials node)

- render a default cube (i'm in Cycles)
- jump to node editor, check use nodes and backdrop, (add a viewer if it's not there)
- create 2 or 3 nodes and group them (color correction, masks, is irrelevant)
- once created you autojump in the new group, inside this new group all nodes are selected, select one (to deselect all the others basically) , then undo --> crash, everytime for me.