Hubble uses what is called its Fine Guidance System (FGS) in order to maintain stability while performing observations. A set of gyroscopes measures the attitude of the telescope, which is then corrected by a set of reaction wheels. In order to compensate for gyroscopic drift, the FGS locks onto a guide star as a fixed reference in space.
In this case, Hubble locked onto a bad guide star, probably a double star or binary, causing an error in the tracking system. The result was this remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288.
Image Credit: NASA / ESA