I started to review the logs in BIND and noticed that there are entries for server failure error code 2.

1) Why is causes these errors, even if the URL queried are valid?
2) How can I investigate the cause of these errors?
3) Is this a normal happening in BIND? I am seeing about 0.5% error rate.
4) What is considered normal rate?

Anyone has any statistic showing the max query per second that can be handled by BIND?