What happens if rsnapshot / rdiff-backup gets interrupted in the middle of a transfer?

My understanding is that...

rdiff-backup will detect the incomplete increment the next time it runs. It will delete the incomplete increment so that the backup location is the same as if the interrupted backup attempt had never been started.

rsnapshot is a little more complicated because its routine is more stepwise and varies depending on the use of the sync_first and use_lazy_deletes options.

• If you use sync_first and rsnapshot sync gets interrupted, you can simply run rsnapshot sync again to straighten things out. If you accidentally run rsnapshot <backup level> at this point instead, the latest backup point will remain incomplete and will be carried through rotations.
• If you don't use sync_first, you're just stuck with an incomplete backup point that is a hybrid of old and new version of files. Unless you manually reverse-rotate each backup point, the incomplete backup point will be carried through rotations.
• In both cases, running rsnapshot <backup level> will cause the oldest backup point to be lost unless use_lazy_deletes is enabled.

Note that sync_first and use_lazy_deletes come at the cost of using more disk space.

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A reminder/disclaimer: This should go without saying, but never just blindly trust others' advice on the internet. If you plan to use rdiff-backup or rsnapshot for something mission-critical, read every word of the manual and test, test, test everything yourself!