Stirling Newberry writes: "This image shows "P5," the place holder name for a 5th natural moon of Pluto, a tiny sliver that orbits ~29,000 miles from its primary in a circular orbit. Other than Charon, Hubble has been the means of finding all of the known moons of Pluto."
If it happens once, it's a bug. If it happens twice, it's a feature.
If it happens more than twice, it's a design philosophy.