You have to be able to recognize when a rarely used mode produces data that are not routine. You have to be able to tell what features are intrinsic to the target. You have to recognize known artifacts for all instruments and their many modes. Anomalies may be either routine or serious. A serious anomaly is one that may be threatening to the health of the instrument and to the quality of future observations.
Serious anomalies are hard to categorize, since they are unique events.
Furthermore, predictions of what software or hardware component will fail are unlikely to be accurate. It is necessary to rely on your experience in examining HST observations to determine what is routine and what is indicative of a failure. Typically, routine anomalies repeat from time to time, and are benign. A routine anomaly may have been classified as serious when it first appeared. After the engineers have analyzed the effect and shown that it does not threaten the health of the instrument, it will be classified as an effect that should be noted and reported, but for which no emergency action is required.