Comment 7 for bug 1286542

I've experienced this on my custom server as well, even though the deltas are generated localy, so I started digging - the corrupted index.json was caused by this code in s-i-server[1]. It causes previously generated deltas with the correct target and destination hash to be accepted even if target and destination hash are swapped.
As you can see, it is support for some old wrong code which produced badly named deltas, so maybe it can be removed. I'll try to remember to ping stgraber about it once he's back from his vacation.

[1]: http://bazaar.launchpad.net/~ubuntu-system-image/ubuntu-system-image/server/view/head:/lib/systemimage/generators.py#L91