This team pushed multiple changes to prod per day. And the load balancer with autoscaling is bouncing instances on its own. And, and, and...
So resource leaks were being masked. It was only noticed during a lull in work.
And then because so many releases had passed, delta debugging was a lot tougher. And then because nodejs & npm has a culture of one module per line of source (joking!), figuring out which module to blame took even more time.
I think continuous deploys are kinda cool. But not without a safety net of some sort.
One of the companies I worked at had an incident a couple years before I started, where there were multiple malicious Python libraries being used in the code. For 3 months.
Luckily, the libraries didn't do anything significant except to ping an IP address in China, actually did perform their advertised functionality, didn't exfiltrate any data other than the source IP address on the ping packets, and they were easy to replace once the situation was found out. But, for months, they had servers pinging or attempting to ping somewhere in China.
Oh, and those libraries made it into our internal repos we were using to pin versions with, too.
Beyond being a speecy spicy meatball of a story, the moral here is that you have to constantly be on your guard and verifying every single line of code that goes into your application somehow.
Just attempting should create alarms.
Also, you should try to update as few things as possible at once, and let your changes "soak" into production for a while before going in all "upgrade ALL the things!"
Why? Well, sometimes things that fail take a while to actually start showing you symptoms. Sometimes, the failure itself takes a while to propagate, simply due to the number of servers you have.
And, one of these days, cosmic rays, or some other random memory/disk error is going to flip a critical bit in some crucial file in that neato library you depend on. And, oh, the fouled up build is only going to go out to a subset of your machines.
You'll be glad you didn't "upgrade ALL the things!" when any of these things ends up happening to you.