There seems to be a moment in a day or every few days, where the ObjectId.GenerateNewId() generates a 23 length ID, and fails to Parse.
We noticed this error live, and I was able to reproduce this error in leaving a test running all day, where i would call ObjectId.GenerateNewId(), and parse it to an ObjectId. It seems to randomly return a 23 length ID at different times on different days, which of course wont parse.

The error and a sample Invalid id:
System.FormatException: '590262767c771f2b986a59b' is not a valid 24 digit hex string.
at MongoDB.Bson.ObjectId.Parse(String s)