If an invalid collection name like "$cmdBREAK" is used for out in map_reduce, an index could be created for that collection even though the collection won't be created. I've only found this to be a problem in versions of mongodb below 2.0.9. In the versions <= 2.0.9, an index and database will be created even after the current driver says "pymongo.errors.InvalidName: collection names must not contain '$':". Using the example in the comments causes the db $cmdBREAK to appear although it won't show up after "show collections".