As I remember, normally #2 was shut down after landing, saving brakes, provided the electric hydraulic pumps were on. Soon after these pumps were deemed unusable, a crew taxied in at Heathrow with all 3 running; as per habit, the Captain shut down 1 & 3 upon block-in, forgetting #2. It ran, unobserved, for 3-4 hours, as its high position & other noise (APU's, etc) made its running almost invisible, except for tape instruments in the cockpit. Rumour has it that, when this was finally discovered, the Captain was called out of rest to shut down the engine.