Steps to reproduce:

> db.test.foo.insert({"a":

{"c0":1}

, 'c':2})
> db.test.foo.find(

{"c":2}

)
{ "_id" : ObjectId("4def775276aed51aaa0684d3"), "a" :

{ "c0" : 1 }

, "c" : 2 }
> db.test.foo.update(

{"c":2}

, {'$inc':{'a.b0':1}})
> db.test.foo.find(

{"c":2}

)
// So far, so good
{ "_id" : ObjectId("4def775276aed51aaa0684d3"), "a" :

{ "b0" : 1, "c0" : 1 }

, "c" : 2 }

//Things go off the rails. This should add a new field, 'c00'
> db.test.foo.update(

{"c":2}

, {'$inc':{'a.c00':1}})
> db.test.foo.find(

{"c":2}

)

// Notice the 'c0' key has been changed to 'c00'. This should have added a new field.
{ "_id" : ObjectId("4def775276aed51aaa0684d3"), "a" :

{ "b0" : 1, "c00" : 2 }

, "c" : 2 }

// This works as expected.
> db.test.foo.update(

{"c":2}

, {'$inc':{'a.cc':1}})
> db.test.foo.find(

{"c":2}

)
{ "_id" : ObjectId("4def775276aed51aaa0684d3"), "a" :

{ "b0" : 1, "cc" : 1, "c00" : 2 }

, "c" : 2 }
>