The file zips.json is mentioned here:

http://docs.mongodb.org/manual/tutorial/aggregation-examples/

The data from this file are used in the examples.

This file contains duplicate _id keys listed below:

1290:

{"city": "LITTLE ROCK", "loc": [-92.422232, 34.787076], "pop": 16183, "state": "AR", "_id": "72212"}

4016:

{"city": "PINETTA", "loc": [-83.340463, 30.599703], "pop": 642, "state": "FL", "_id": "32350"}

5352:

{"city": "PINETTA", "loc": [-83.310491, 30.634794], "pop": 0, "state": "GA", "_id": "32350"}

6916:

{"city": "SAINT MARY", "loc": [-89.929268, 37.914034], "pop": 130, "state": "IL", "_id": "63673"}

9909:

{"city": "FORT CAMPBELL", "loc": [-87.459706, 36.653584], "pop": 18861, "state": "KY", "_id": "42223"}

13771:

{"city": "FLORA", "loc": [-90.323506, 32.557832], "pop": 3973, "state": "MS", "_id": "39071"}

14215:

{"city": "SAINT MARY", "loc": [-89.976989, 37.832481], "pop": 2730, "state": "MO", "_id": "63673"}

14652:

{"city": "KAISER", "loc": [-92.579869, 38.163673], "pop": 1213, "state": "MO", "_id": "65047"}

24521:

{"city": "FORT CAMPBELL", "loc": [-87.558507, 36.599497], "pop": 728, "state": "TN", "_id": "42223"}

28045:

{"city": "BLOOMINGROSE", "loc": [-81.636737, 38.141189], "pop": 191, "state": "WV", "_id": "25024"}

The utility program mongoimport silently ignores duplicates.
Which duplicate would be ignored could not be predicted, I believe.

For example I aggregated total population as: 248706415 and 248690240.
Some aggregation results presented on the above page could differ too.

I believe that this data could be easily corrected by an US citizen.

--Włodek Bzyl