The first major backup of 6.5TB had only a single apparent failure:
time="2015-07-31T19:32:44-07:00" level=fatal msg="Sync failed: createFile: setMetaData chown: chown /mnt/backup/data/home/demizer/code/code/c/pacman/lib/libalpm/util-common.c: no such file or directory"
util-common.c is a symlink, so the problem could be that chmod is following the symlink but the file it is pointing at doesn't exist.