The interesting bit is this: how much of what you wrote over all those years actually did what you wanted it to do, no more, no less?

This is where testing gets interesting: I took some old code I wrote 30 years ago or so and decided to put it literally to the test. A couple of hundred lines from a library that has been in production without every showing a single bug over all that time. And yet: I'm almost ashamed at how many subtle little bugs I found. Things you'd most likely never see in practice, but still, they were there. And then I put a couple of those bugs together and suddenly realized that that particular chain must have happened in practice in some program built on top of this. And sure enough: fixing the bugs made the application built on top of this more robust.

After a couple of weeks of this I became convinced: testing is not optional, even for stuff that works. Ever since I've done my best to stop assuming that what I'm writing actually does what I want it to. It usually does, for the happy path. But there are so many other paths that with code of any complexity, even if you religiously avoid side effects you can still end up with issues that you overlook.