I end up in a lot of incidents where we spend time trying to track down why there has been some small number of failures in a particular piece of code. I like a good bug hunt, but I think this is almost always the wrong question to ask.
The thing is, there are just so many reasons why something can go wrong. Things can go wrong because you have written bugs. Things can go wrong because you have followed the spec too literally. Things can go wrong for boring and yet unavoidable reasons. Things can go wrong when you get so cosmically unlucky that you would never believe it if you hadn’t seen it with your own eyes.
But the surest thing of them all is that if you’re running software at large enough scale to have a software engineering department things will go wrong. Even if you figure out what it was this time it’ll be something else next time. Your time is better spent thinking about how you’re going to handle it when that happens.