import pymongo
conn = pymongo.Connection()
coll = conn.test.create_collection('bar', autoIndexId=False)
coll.update({'a': 1}, {'$set': {'b': 2}}, upsert=True)
list(coll.find())
[{u'a': 1, u'b': 2}] # NO _id!!

Once _id is missing we saw fatal assertions coming from replica set code (we tried creating the index but it complain on dup key error) and failed migration because migrated documents must contain a _id. I assume all documents in mongo must have a _id.