Hi,

this error is very rare (I cannot reproduce it), it happens on production every one-two hours (operation which cause this errors have rate ~100/s).

I read about this issue but every topic I found is about multiple unique indexes which isn't the case here.

My collection has one unique index (_id), two regular ones and one Map Field(EmbededDocumentField).

I think these issues started after upgrading Mongo to 3.0 and WiredTiger engine.

Current setup:
MongoDB 3.0.8
PyMongo 3.2.0

Sample query which causing issue:

collection.update({'_id': '9737600500733084563'}, {'$inc': {'fields.123.counters.33' : 1}}, upsert=True)