I've been updating my website with "rsync -a --delete (from) (to)" for years. Very happily so, I might add. Only this time, the original happened to be on a local NFS mount, and that mount point happened to be inaccessible when I did the update.

So rsync then deleted hundreds of MB's from my webserver before I could hit CTRL/C ...
(Many hours later: all the data has been pushed back out through the ADSL uplink. Phew!)