起因是因為 iOS 上 safari 瀏覽器在做 ajax request 的時候，有時會有bug (https://stackoverflow.com/questions/33895463/safari-ajax-request-failed-to-load-resource-the-network-connection-was-lost)，有個解法是把網頁伺服器的 keep-alive 關閉

reference:
https://manage.togglebox.com/index.php?/knowledgebase/article/20/turn-off-keepalive-to-reduce-apache-ram-usage/
https://svennd.be/keepalive-on-or-off-apache-tuning/