A method for testing a computer program that includes software code arranged to be executed in multiple threads, including at least a first thread and one or more other threads. The method includes identifying an output statement in the first thread, running the software code so as to generate an output due to the output statement, and intervening in the program so that when the program is run, execution of the first thread is blocked prior to the output statement while the one or more other threads continue to execute. The output generated after intervening in the program is examined in order to detect a fault in the program.