If you shard a collection with a key like {{

}} the system stores that as a field name in various collections (collections, chunks, changelog and tags to name a few).

This causes problems with backing up and restoring data or moving data to other databases (since these fields are invalid as user documents). It also means that the clients/driver and server has to special case these collections (or database) to disable regular validation (hence the problem moving this data to user collections).

This is somewhat related to trying to copy the profiler documents (which have fields with $prefix or dots) to different collections.