Ignoring null in embedded groups, this is produced when a element is removed from the group and saved, check mongodb issue:
https://jira.mongodb.org/browse/SERVER-1014

The workaround is using $unset and then $pull, but this can break the concurrent operations:
db.lists.update({}, {$unset : {"interests.3" : 1 }})
db.lists.update({}, {$pull : {"interests" : null}})