JMeter keeps track of all errors that occur during a test in a log file named jmeter.log by default. The file resides in the folder from which JMeter was launched. The name of this log file, like most things, can be configured in jmeter.properties or via a command-line parameter, -j <name_of_log_file>. When running the GUI, the error count is indicated in the top-right corner, that is, to the left of the number of threads running for the test, as shown in the following screenshot. Clicking on it reveals the log file contents directly at the bottom of the GUI. The log file provides an insight into what exactly is going on in JMeter when your tests are being executed and helps determine the cause of error(s) when they occur:
JMeter GUI error count/indicator
