Negative yields have been very common in the last 2-3 years, perhaps not when the bond is initially sold, but certainly after the bond starts trading - when the negative yield also takes into accounts expectations around currency risk. A currency that is expected to hold it's value (like the US Dollar) - is a very desirable place to park your money if you believe other currencies will drop. And, as you purchase those bonds, and drive up the price, you drive down the yield.

So, while negative yields on savings accounts are pretty new and strange thing, negative yields on bonds aren't entirely unexpected.