If this php error is occurring intermittently then this is completely normal/typical. If this php error is occurring constantly then that would not be normal. Things that can cause a WordPress Database query to not complete successfully.
Too many DB query requests at the same time.
Temporary Server connectivity issue.
A totally random php processing error or Server hiccup.
A bad/malformed query was sent to the DB.
Please read this php error Read Me First Forum Topic to find out which php errors matter and how to handle php errors in general: http://forum.ait-pro.com/forums/topic/how-to-troubleshoot-php-errors-php-errors-in-your-php-error-log/