In migrating from 2.11.2 to 2.12.0, we've encountered a multitude of cases where we were using:

dbCollection.ensureIndex(new BasicDBObject("foo", 1), "someName", true);

Doing the find/replace 's/ensureIndex/createIndex/g' wouldn't be bad, but not having a matching 3-arg createIndex(spec, name, unique) makes migrating more manual and error prone.