From a high level point of view, the addShard method triggered by a _configsvrAddShard command is executing the following steps:

1) Check if the shard exists in config.shards (if yes, return).
2) Write a new document representing the shard into config.shards.
3) For each database on the shard, write a new document representing it into config.databases.

If there is an interruption between steps 2 and 3, any addShard retry will not execute step 3 resulting in config.databases potentially presenting an inconsistent/incomplete state.