Your WordPress site sometimes shows ‘There has been a critical error’ and then starts working again on its own, with no deploy or plugin change behind it.
Why this happens. An error that appears and clears by itself is usually not a fault in your site’s code. The common cause is a request that takes longer than the server’s PHP time limit and is killed mid-page. WordPress reports this as a critical error.
How to diagnose it. Check the PHP error log for lines reading ‘PHP Fatal error: Maximum execution time of N seconds exceeded in wp-includes/class-wpdb.php’. That message means the page was still waiting on a database query when the time limit ran out. It points at how long queries are taking on the server, not at a broken plugin or theme.