The bugs occur because the initial tests didn’t fully capture the desired and undesired behaviors.
I’ve never seen a formal list of software requirements state that a product cannot take more than an hour to do a (trivial) operation. Nobody writes that out because it’s implicitly understood.
Imagine writing a “life for dummies” textbook on how to grow from a 5yr old to 10yr old. It’s impossible to fully cover.
Hah, if that were true the industry would be a better place. Or a worse place. Or a slower place but exactly the same. I should build a test for that...
I've worked on many projects where tests get disabled as nobody can tell why it's failing (or why it was even written in some cases).
I've rewritten test systems from scratch in the past to drag projects out of the dumpster fire by getting them into a state of passing simple startup/shutdown safely routines and then watched as I pass the project onto others how it rots until some "genius" young coder comes along and "removes the slow test-suite because it takes 2hr+ to run on my way out of spec laptop".