We recently had an issue where an index did not get created on all shards. When we ran getIndexes() on mongos it reported the index was present. So we dropped another redundant index thinking all would be well - it was not. We had serious performance issues on the shard that was missing the new index and our app was unavailable for a few hours whilst we re-created the missing index on a huge collection.

It would be better if getIndexes called on mongos reported some sort of warning or indication that it was not present on all shards. The same should also be clear on mongo cloud's index view. Even better would be some guarantees that an index will get distributed to all shards, even if there is a transient problem with one of the shards.