An airline I was once acquainted with had a fleet of 737-200's. Soon after delivery Boeing realise a vital pin in the wing to fuselage joint was not to the latest production spec on some aircraft, so they issued a Service Bulletin listing the effected airframes. Boeing would supply the correct pins FOC, to be fitted at the first heavy maintenance input. A mod was raised, the parts ordered and on arrival they went into stock. Some six years later they were needed but had gone, disposed of in a stock reduction scheme as being none moving stock, sold to a parts trader who had sold them on to a less efficient operator who had not even ordered them when they were FOC. It cost a fortune for a small batch to be made.