Deflation of any note! How about 2009 compared to 2008, at least according to CPI. That is why not SS increase in 2010 because of the CPI decrease which would have resulted in a reduction except the SS rules is that the CPI adjustment to SS is never negative. Some of my inflation protected Series I savings bonds, went from 6% to 0% for 6 months which had a rate of 3% + CPI for each 6 months. Since 6 mo. CPI increase was -5%, that added to 3% was a negative number. However that also was controlled to 0% minimum.
How did it happen? Well 2008 had an energy cost bubble (think gasoline over $4, crude over $100/barrel) that went away in 2009, and there were rent reductions in 2009 according to the Department of Labor. The CPI used is technically CPI-U which is supposedly the cost of a typical urban worker's expenses. If interested, go to the Bureau of Labor Statistics, it is quite technical.
This may not meet the definition "of any note", but according to CPI figures these things happen. Actually, in the 6 month period ending in November 2009 inflation is again positive and my Series I bonds are now, or soon will be positive, and new Series I savings bonds pay more than bank CDs.
-zzyp-