Luckily this one was caught while still in development as there were more than 20 old "snapshots" to test with: rsync: ERROR: at most 20 --link-dest args may be specified Looking into the source: rsync.h:151:#define MAX_BASIS_DIRS 20 There is no warning about this in the rsync manpage. This limit should be lifted as it makes no sense to me.
The limit is now documented.