Steps to reproduce:
1) create unique index:
test.ensureIndex(new BasicDBObject("field", 1), "fieldIndex", true);
2) save object

{field : 1, d : 1}

3) save object

{field : 1, d : 2}

Expected: exception on step 3
Detected: no exception. first object stored in DB.

Unit test:
@Test
public void testUniqueViolation() throws UnknownHostException {
Mongo mongo = new Mongo();
mongo.dropDatabase("testdb");
DB db = mongo.getDB("testdb");
DBCollection test = db.getCollection("test");
test.ensureIndex(new BasicDBObject("field", 1), "fieldIndex", true);

test.save(new BasicDBObject("field", 1).append("discriminator", 1));
try

{ test.save(new BasicDBObject("field", 1).append("discriminator", 2)); Assert.fail(); }

catch (MongoException e) {
}
}