Deflation

Krugman on deflation:
inflation and deflation are not symmetrical. Four percent inflation does very little harm; four percent deflation is a disaster. Why? Three reasons:

1. The zero lower bound: you can always raise interest rates, but you can’t cut them below zero, so deflation means significantly positive real rates even in the depths of a slump, making monetary stabilization much harder if not impossible.
2. Nominal wage rigidity: it’s hard to get wage cuts — always has been, and always will be. So deflation messes up labor markets.
3. Debt: deflation is always contractionary, because it redistributes wealth to creditors and away from debtors, who are almost by definition more likely to be spending-constrained. And in the euro context this means that imposing deflation on debtor countries worsens the downward pressure on the European economy.
So European policy that requires deflation on the part of a large part of the zone is a real disaster.

And there is more.
4. The problem in a recession is that people want more money and less goods and services.  Deflation makes consumer, want to wait to buy stuff because prices are coming down and hoarding cash becomes even more attractive as it is going up in value.
5. Banks become less interested in lending money when they can earn a real interest rate by simply holding cash rather than lending it out to someone who may not pay it back as the real debt burden of a loan compounds with the deflation rate. The increased real interest rate burden of a loan (nominal interest rate plus the absolute value of the deflation rate) also makes borrowers more hesitant to borrow. 
6. (related to #3) Deflation can increase bankruptcy rates which reduces the spending of both the debtor and the creditor until it is sorted out which can take a long time.  Many simultaneous or big bankruptcies can paralyze a financial system and bring down an economy.  This was the fear during the 2008 financial crisis.  Even in the long run, bankruptcy is an expensive process with high transactions costs that reduce the productivity of an economy.