downward nominal wage rigidity

Potential Mistakes (Wonkish) by Krugman

But right now we have high unemployment combined with more or less stable core inflation. Typical models would interpret this as a sharp rise in the natural rate, from maybe 5.5 to 8 percent. But what it almost surely reflects instead is the stickiness of inflation at low levels; the long-run Phillips curve isnot vertical thanks mainly to downward nominal wage rigidity,and that reality is central to what’s happening now. 

I wish that these were narrow technical issues, of no importance for real-world policy. Unfortunately, they’re not. Understating output gaps leads to excessive demands for austerity and excessive complacency at central banks; this perpetuates the depression; and the longer the depression goes on, the more misleading the standard estimates become. 

So it’s good news that at least somebody in Brussels is aware that there might be a problem.