My co-author Paul described an interesting failure mode for a 7805 regulator IC, which was in a circuit with a modest load current, and with about 1uF of load capacitance.
The failure symptom is that when 12V power is first applied, the output voltage first goes to 12V, before dropping back to 5V and operating properly. When the power is turned off, the output voltage again spikes high.
One possible clue: when the 7805 was first used in this circuit, a hand-wired prototype, the ground pin was left floating. But when the failure mode was observed this pin was properly grounded.