From a recent question in training: "what happens if you try to use the positional operator with upsert?". It appears it can create an invalid document with a dollarsign in the field name:

db.foo.update(

{ 'x.a' : 6 }

, { $set :

{ 'x.$.b' : -5 }

}, true )
db.foo.find().pretty()
{
"_id" : ObjectId("4f1724449afa61972cf8ef0b"),
"x" : {
"$" :

{ "b" : -5 }

,
"a" : 6
}
}