Rsync problems when syncing large files

Speaking of rsync, I've noticed that it does not work reliably when syncing virtual disk images. In particular, when performing backups or migrations and an older version of a disk image exists I find that rsync does not correctly copy the changes. I have seen this several times now, each time verifying with an md5 and sha1 hash. I do notice that the syncs work correctly so long as the '-c' (checksums) option is used. Of course that option slows the process considerably. I'll be interested to test rsync 3 to see if this behavior is improved.