Sample code:

    function test_error_and_exception () {
        count(); // results in warning
        throw new exception('buuuuaa'); // this exception nukes simpletest
    }

    function test_fails_with_exception () {
        return;
    }

fails in both tests with:

Exception: lib/dml/simpletest/testdml.php / ? dml_test / ? test_error_and_exception
Unexpected exception of type [Exception] with message [buuuuaa] in [C:\workspace\moodle\lib\dml\simpletest\testdml.php line 65]

Exception: lib/dml/simpletest/testdml.php / ? dml_test / ? test_fails_with_exception
Unexpected PHP error [count() expects at least 1 parameter, 0 given] severity [E_WARNING] in [C:\workspace\moodle\lib\dml\simpletest\testdml.php line 64]

working on a temporary patch ;-)