I just went to rsync my desktop in the UK with the backup drive from France and got the command backwards, so ended up with the old UK files overwriting the new France files on the backup drive!
Luckily I correctly rsync’ed my $HOME directory first, so its only downloads and backups that got nuked. I can live without them for a week. My VM’s and work/office stuff are fine.
Note to self: next time proof-read the command before running it as root, and mount the backup drive readonly!