I guess we had the same inversion in the 1-month and 3-month during the November 2007 market slide of about 9%. Maybe the 1-month T-Bills are trying to tell us something:
In edit, other than the Feb. 2008 and Nov. 2007 time periods, I can't find any other examples going back to 1999 where we had such a large inversion in the 1-month and 3-month T-Bills