How can you monitor for hidden issues while performing tests? In the case of the web application, QA engineer can use browser dev tools console to see error traceback.
Also, it is really useful to handle all exceptions with some issue tracking tool, for example, Rollbar or Sentry. So if something goes wrong, a team can get notifications via email/ SMS, or chat (depends on how it was set up). It provides detailed error information: traceback, browser/OS, URL, IP Address where it occurred, etc.
It’s important to use monitoring software tool that provides monitoring metrics, like server availability, CPU load and disk space consumption. If some application goes down, the team can be notified via email/ SMS, or chat. For example, open source Zabbix.
In addition, there are a lot of different solutions that check if the website is available and running. These tools help to ensure proper alerting is in place, so the team gets notified when the website is not available. For example, Pingdom or Uptime.
Performance testing from a UI perspective is also crucial because no one likes to wait a long time to see page load. So one of the ways to ensure that UI speed was not downgraded is to use some monitoring tools of page load(like CalibriApp). The other way is to keep an eye on UI tests runtime. Also, it can be handy to implement load tests into CI and have a statistics about application performance after each commit. So if there is a big downgrade after some updates, the team should consider investigating its causes.