The Pioneer anomaly, one of the longest-standing mysteries in physics, has apparently been solved. It seems someone forgot to carry the one :-)
Actually, that's not so far from the truth. The problem with earlier models turned out to be that they didn't take into account the way heat from the spacecraft's plutonium power source reflected off the antenna.
What is most mind boggling to me is that it's actually possible to measure the position of the spacecraft accurately enough to tell the difference.