Requires hacking codebase: 1) Edit mod/chat/lib.php and, in the  chat_login_user()  function, exactly before the chat_send_chatmessage() call, add a 2 seconds sleep. } else { + sleep(2); chat_send_chatmessage($chatuser, 'enter', true); } 2) Initialise PHPUnit: php admin/tool/phpunit/cli/init.php 3) Run the chat unit tests (100 repetitions, yes 200 seconds, be patient): $ vendor/bin/phpunit --repeat 100 "mod_chat_lib_testcase" mod/chat/tests/lib_test.php 4) Verify that execution ends without error. 5) Optional: Before the fix and with the sleep() manually added in 1) it was consistently causing an assertion to fail with a 2 seconds difference. You can verify it if you want, without the fix applied.