Application errors often do not result in erroneous HTTP responses. More commonly, applications respond with incorrect data values or error messages that are incorporated into HTML content, such as Servlet Exception Occurred or Server Too Busy error messages. Because checks of HTTP status code do not uncover this class of error, application errors are often overlooked unless verification functions that check for non-standard HTTP errors are built into test scripts.
When verifications are built into test scripts, tests go from being simple load tests to hybrid load/functionality tests. Such scripts can be utilized without incurring significant performance loss, even in large load test scenarios. This functionality allows you to detect a class of error that other load tools are unable to detect because errors that occur only under load are undetected with standard load test scripts.