Ask HN: How do you look for rare log patterns and RCA?

Hey, I'm just curious. I know that there is a feature in Datadog that shows new log patterns formed and disappeared. So when something goes wrong, what is it that developers look for beyond this? It should be pretty straightforward to find an error in the haystack. By looking at novel error patterns, right?

When was the last time you had to spend hours debugging to find the root cause?