This is one of the most interesting questions raised by the fleet of unmanned spacecrafts floating around (and outside) the Solar System. In short, the Pioneer probes are not where they should be, and nobody knows why.
The two Pioneers (10 and 11) were launched in late 1972 and early 1973 and are currently two of the most distant man-made objects in existence. Neither of them is active anymore; the last signal from Pioneer 11 arrived in 1995, and from Pioneer 10 in 2003. They are still moving away from us in more or less opposite directions: 10 is heading to Aldebaran, which it should reach in about 2 million years, and 11 will fly by the constellation of Aquila in 4 million years.
Being this far away from us and having travelled for such a long time, they are in an excellent position to show us unexpected news about the way the universe works, and they may have done just that. Their actual position over the last few years was consistently "wrong" when compared to their expected position; the error indicated that they were being subjected to a small force towards the Sun that exceeds what should be expected from gravity and other known effects.
Nobody knows what that force is, of course. It could be something as simple as a gas leak, but it would be definitely strange that this would cause exactly the same amount of force applied in the same direction in both probes (then, again, they are identical; if it's a design problem or side-effect, it's not so far fetched as it sounds). It could also be an observational error; old data is being revisited to try to rule this out.
We do have different probes that are as far away as the Pioneers: Voyagers 1 and 2 are still operating and have already left the outer-most planets behind; Voyager 1 is, in fact, farther away from the Sun than any other probe (Pioneer 10 trails it by almost 1 billion kilometres, and Voyager 2 by 3 billion). However, the Voyagers are too different from the Pioneers: their stabilization system is different, and it causes thrusters to fire quite often to keep them pointed in the right direction; these thrusters interfere with the trajectory of the ships and, thus, with the very small effect of this unknown force. Still, they seem to be affected in the same way, although data is not quite conclusive. The probes Galileo and Ulysses, which are stabilised in the same way as the Pioneers, never ventured so far away from the Sun that the effect on them could be conclusively measured.
It is expected that New Horizons, the new mission to Pluto, planned to be launched early next year and to reach Pluto in 2015, could bring some light to this issue; as the Pioneers, it is spin-stabilised and should be affected by the same forces. Whatever the reason for the anomaly turns out to be, and even if it is indeed something quite prosaic, we will certainly learn something new: either about some aspect of space flight that was previously overlooked, or about some new facet of the inner workings of the universe.