The bson.ErrTooSmall error is used in many places, including being returned from deep recursive calls. This makes it extremely difficult to track down the stack that is generating the error. Make ErrTooSmall an error type that includes a stack trace when it's generated. Then change all the places where ErrTooSmall is used to create a new ErrTooSmall that includes a stack trace. This will greatly help in debugging problems with sizing.