To examine a non-recurring specific issue, such as all sessions from yesterday that had errors, or the last 1000 pages with server errors, you must run a custom query in the Session Browser.
In a Real User Analyzer, open the Session Browser tab.
In Collector feed name, limit the results to show traffic from one of the Real User Collector components associated with the Real User Analyzer, or choose any.
To analyze the query result, proceed with detailed query analysis.