This typically happens if you install Notes/Domino optional components inconsistently. E.g., you installed Notes 4.61 with Advanced Services, but then you didn't with 4.62. When you try to install 4.63, it detects some old files from the 4.61 install that are not at the 4.62 level (failing a checksum). You can see which files these are by looking at the UPGRADE.LOG file in the Notes directory. Usually, you can keep deleting these old files and re-running the incremental installer until it works (the installer stops when it detects the first file that fails the checksum).