To reproduce the bug, I run these 3 commands:

db.bugtest.insert(

{ _id: ObjectId("4c8d156f6709000000001176")}

);
db.bugtest.update(

{ _id: ObjectId("4c8d156f6709000000001176") }

, { $set:

{ "babca304b2c79a6801200000": "Test 01" }

});
db.bugtest.update(

{ _id: ObjectId("4c8d156f6709000000001176") }

, { $set:

{ "babca304b2c79a6801210000": "Test 02" }

});

I expect this object:

{ _id: ObjectId("4c8d156f6709000000001176"), "babca304b2c79a6801200000": "Test 01", "babca304b2c79a6801210000": "Test 02"}

Instead, "babca304b2c79a6801200000": "Test 01" is missing:

{ "_id" : ObjectId("4c8d156f6709000000001176"), "babca304b2c79a6801210000" : "Test 02" }