it's easy to pass an object instead of an array by mistake. returning an error would be ok but adding something containing "$each" as a field is bad. same check needs to happen if the value is other types.

> t.update({},{ $addToSet : { a : { $each :

} } })
> t.find()
{ "_id" : ObjectId("4eeb4609711dd7183d441894"), "a" : [ 3, 5, 6, { "$each" :

} ] }