When connecting to MongoDB 3.0.8, occassionally the call to MongoClient.connect will fail silently. The callback will never get invoked. I've traced it and it appears internally that the same thing is happening when the driver attempts to authenticate with the database. Reverting to 2.2.33 seems to solve the problem. Happy to provide more information.

It appears it may be a timing issue, because adding delays (or breakpoints) while debugging seems to make the problem much less frequent.