When running the update installer, it failed partway through the diff comparison with the error message mentioned in the KB article The installer detected a timeout. Its recommendation to check the logs for a catch-all error were indeed correct and led me to the 3.3.12 correction in includes/diff/diff.php:

                if (is_array($orig) && count($orig) == 1 && trim($orig[0]) === '')

                {

                    $orig = array();

                }

                if (is_array($final) && count($final) == 1 && trim($final[0]) === '')

                {

                    $final = array();

                }

I asked about this problem here. For the next person with this [timeout?] problem, I recommend adding a link to the above KB article in the message LA_TIMEOUT_DETECTED_MESSAGE, e.g., "For other possible causes of this error, see the article The installer detected a timeout".