Unique IDs generated with the Java MongoDB driver do NOT conform to the documentation both mongoDB generally and the javadoc included in the org.bson.types.ObjectID.

In particular, machineID and processID is 4 bytes, not 5 and Increment is 4 bytes, not 3, as would be expected and is documented. Knowledge of the precise structure is relevant when the _id is being manually generated or specific bytes modified to still ensure (distributed) uniqueness. In short: Documentation does not match implementation.