Little bit of head scratching fun today.
A server that was spun up with a restored snapshot did not update a 3rd party repo.
I found out that the DO folks assume /etc/apt/sources.list are theirs for the taking and clobbering upon a restore (independent of region).
So, even though they don't "control" your setup, they do put their finger in the works a bit on restore or install from their created base images.
They are probably mostly right as 3rd party repo adds should happen in sources.list.d dir as an additional sources list.
If you provide repos to customers using Debian / Ubuntu, you may wish to verify that you are following best practices.
Props to ryanpq in the DO forum
https://www.digitalocean.com/community/questions/snapshot-of-updates-in-sources-list-in-ubuntu14-04-not-saved
Looks like later versions of the sources list calls out cloud-init as the cause (see the header on the file) for ways to avoid issues.