Handling corrupted background data in an out of order processing environment. Modified data is stored on a byte of a word having at least one byte of background data. A byte valid vector and a byte store bit are added to the word. Parity checking is done on the word. If the word does not contain corrupted background date, the word is propagated to the next level of cache. If the word contains corrupted background data, a copy of the word is fetched from a next level of cache that is ECC protected, the byte having the modified data is extracted from the word and swapped for the corresponding byte in the word copy. The word copy is then written into the next level of cache that is ECC protected.