when interrupted in between trying to persist new chunk changes or when the changes gets partially rolled back.

When shard tries to persist new updates to config.cache, it sets the refreshing flag, deletes the current chunk document and then inserts the newer version. And finally unsets the reloading flag after it finishes processing the updated chunks. However, if the mongod crashed in the middle or some of these writes get partially rolled back (such that insert was rolled back, but not the delete), the config.cache will now be in an inconsistent state.