While normally changelog would be excluded because it lives in config database, it is very easy to restore the config DB into a differently named database and get an error from upgradeServer() check on every single document in that collection that has a shard key with . in the name

(example if I shard on "foo.bar" where bar is embedded document inside foo, like "user.screen_name" in twitter json, for example).

Suggestion: exclude collections named "changelog" or ignore errors on documents in collection "changelog" if the error is embedded inside "min" or "max" top level fields.