The South Korean Navy is upset at the failure rate of its SM-2, Harpoon and Sea Skua missiles. In 23 test firings over the last few years, over 20 percent of the missiles failed to hit their target. Some of the failures were due to operator error, but more of the problems were because of defective spare parts and maintenance issues in general.
Modern missiles are usually stored in a sealed container, which they are also fired from. Sensors monitor the state of missile components, and these are replaced as needed. But after a while, it's time for a refurb, or dismantling and disposal. Generally, this system works well. For example, over the last quarter century, the U.S. Navy has bought over 6,000 Tomahawk cruise missiles, but fired only about 2,000 of them in combat or training. As the older missiles age, they must either be destroyed , or, along with their storage container, refurbished.
Lately, the U.S. Navy has been refurbishing about 250 Tomahawks a year, at a cost of about $200,000 each. It's a lot cheaper than buying new ones. More recent models of the Tomahawk are equipped with sensors that report the status of many components, enabling missiles to be kept in shape with periodic maintenance and replacement of failing components. This does not eliminate refurbishment, but makes the process less frequent and cheaper. This approach typically achieves over 90 percent reliability for the missiles fired. Thus the South Korean officials were upset at the lower reliability rate of their own missiles.