This appeared in SERVER-15013:

It is ambiguous whether "0" refers to a nested field name or to a position in the array. For instance, what should the following projection return?

t.insert({a: [{"0": "foo", "bar": "baz"}, {"a": "b"}]});
t.find({}, {"a.0": 1});

One answer is that "0" should be interpreted as an array position, meaning that we should project just the first array element. The transformed document would then be:

{a: [ {"0": "foo", "bar": "baz"} ]}

Another answer is that "0" should be interpreted just like any other field name, and should be used to project within each nested document. Under this interpretation, the result is:

{ "a" : [ { "0" : "foo" }, {  } ] }

Our projection language chooses the latter interpretation.