For the first time ever, the yield on the 3-month US Treasury bill fell into negative. In other words, investors were willing to pay the US government a fee for the privilege of borrowing their money.
Maths by Bloomberg:
If you invested $1,000 in three-month bills today at a negative discount rate of 0.01 percent, for a price of 100.002556. At maturity you would receive the par value for a loss of $25.56.
Interesting times.