It's always amazing when I get a bug report from a product that's been running bug free in production for years with minimal changes but some user did some combination of things that had never been done and it blows up.
Usually it's something extremely simple to fix too.
This happens a lot more than one may think especially with products that have lot of features. Some features are used sparingly and the moment a customer uses that feature a bit more in depth, boom. Something is broken.
> especially with products that have lot of features
No kidding. I'm 2 or 3 years into working on a SaaS app started in ~2013 and I still get bug reports from users that make me say "what!? we have that feature!?"
Usually it's something extremely simple to fix too.