Sometimes a specific part is corrupted upon upload, and the uploader will create a new post with a "Part 18 Fix."
If you are looking for a "post" because a link is dead or the file is missing, consider these steps: Gears.5.Hivebusters-CODEX.part18.rar
If you have the file but it's corrupted, search for the MD5 or SHA-1 hash of that specific part to see if a verified mirror exists elsewhere. Technical Note Sometimes a specific part is corrupted upon upload,