>so now you have the image plus god-knows-what.

Rsync is smart enought to figure out how to get parity.

>What version of the app is running on which instance?

There is always going to be a time during a rollout where there are mixed versions running.

Sure, rolling forward you expect mixed versions.

Nobody expects a month old version to occasionally turn up and then mysteriously disappear when some rsync job catches up.