rs0:PRIMARY> db.coll.insertOne({"a": {"$foo": 1}})
{
        "acknowledged" : true,
        "insertedId" : ObjectId("5c86835900b48b0b86ee197d")
}
rs0:PRIMARY> doc = db.coll.findOne()
{ "_id" : ObjectId("5c86835900b48b0b86ee197d"), "a" : { "$foo" : 1 } }
rs0:PRIMARY> doc
{ "_id" : ObjectId("5c86835900b48b0b86ee197d"), "a" : { "$foo" : 1 } }
rs0:PRIMARY> doc['b'] = 2
2
rs0:PRIMARY> db.coll.replaceOne({}, doc)
2019-03-11T11:49:14.749-0400 E QUERY    [js] WriteError: The dollar ($) prefixed field '$foo' in 'a.$foo' is not valid for storage. :
WriteError({
        "index" : 0,
        "code" : 52,
        "errmsg" : "The dollar ($) prefixed field '$foo' in 'a.$foo' is not valid for storage.",
        "op" : {
                "q" : {                },
                "u" : {
                        "_id" : ObjectId("5c86835900b48b0b86ee197d"),
                        "a" : {
                                "$foo" : 1
                        },
                        "b" : 2
                },
                "multi" : false,
                "upsert" : false
        }
})
WriteError@src/mongo/shell/bulk_api.js:461:48
Bulk/mergeBatchResults@src/mongo/shell/bulk_api.js:841:49
Bulk/executeBatch@src/mongo/shell/bulk_api.js:906:13
Bulk/this.execute@src/mongo/shell/bulk_api.js:1150:21
DBCollection.prototype.replaceOne@src/mongo/shell/crud_api.js:489:17
@(shell):1:1