The original version 1.3 Painkiller patch released this morning had a corrupted PAK file. The patch has been replaced on the Painkiller Official Website and some mirror sites with the corrected version, but this is one of those Teflon-coated slipstream updates with no increment to the version number, so you'll have to work out for yourself whether you have the right version. The incorrect patch is 94129932 bytes in size, while the corrected version is 94133452 bytes.