New York (CNN Business) Something happened in the bond market last week that has occurred before five of the past six major market meltdowns.

The yield on the benchmark 30-year US Treasury bond — the lesser-known but still important fixed income cousin to the 10-year — briefly dipped below 2.5%. In other words, the 30-year was yielding less than the Federal Reserve's short-term federal funds rate.

It's yet another example of the craziness in the bond market right now. It costs less to borrow money for a period of decades than just a few months — a phenomenon known as an inverted yield curve.

The most widely watched yield curve, measuring the difference between the rates for the 3-month US bond and 10-year Treasury, flipped in late May.

The inversion of the 30-year and federal funds rate has happened only six times since 1980. And five of those times it took place just before a significant pullback in stocks: the double-dip recessions of 1980-1982, the savings and loan crisis of the late 1980s, the Asian debt crisis of 1997, the bursting of the tech bubble in 2000 and the Great Recession of 2008.

Read More