A system and method of providing a cache system having a store-in policy and affording the advantages of store-in cache operation, while simultaneously providing protection against soft-errors in locally modified data, which would normally preclude the use of a store-in cache when reliability is paramount. The improved store-in cache mechanism includes a store-in L1 cache, at least one higher-level storage hierarchy; an ancillary store-only cache (ASOC) that holds most recently stored-to lines of the store-in L1 cache, and a cache controller that controls storing of data to the ancillary store-only cache (ASOC) and recovering of data from the ancillary store-only cache (ASOC) such that the data from the ancillary store-only cache (ASOC) is used only if parity errors are encountered in the store-in L1 cache.