Via FT Alphaville, I see that the US Treasury Borrowing Advisory Committee wants to allow bids for US treasury issuances that have negative interest rates:
The question was asked if it made sense for Treasury to permit bids and awards at negative interest rates in marketable Treasury bill auctions. DAS Rutherford noted that there were operational issues associated with such a rule change, but that the hurdles were not insurmountable. It was the unanimous view of the committee that Treasury should modify auction regulations to permit negative rate bidding and awards in Treasury bill auctions as soon as feasible. Rutherford noted that any decision on this policy change would likely be made at the May refunding.
Fun times.