Small observations can often mean large scale changes in the way we look at the universe. Little semantic things, like whether the Earth goes round the sun or the Sun goes round the Earth.
Small changes in the way we think can have profound impacts on the way we interact, manipulate and predict the world around us.
The Pioneer Anomaly is one such small observation. Since exiting the solar system (inner) and reaching the outer solar system, both the Pioneer 10 and Pioneer 11 spacecraft (launched in the 70's) have slowed down more than any single factor can account for. Like the diminishing of the mass of the Kilogram, it's a small effect that might have big implications about the way we look at the universe.
Further Reading: Planetary.org, Cosmos Magazine