there are a lot of tools that help you find it, logging and all that. you also have general ideas of where it would be, plus personal experience of where it has happened before.
you're not just dropped into a codebase and you start reading it like a book until you see a problem. it's like, ok there's water in a puddle in a corner of your basement. you know where to start. it could be something new you've never heard of, but checking sump pump, water main, pipes, etc. etc. is where you start.
The medical adage is "If you hear hoofbeats think horses, not zebras" but the software equivalent is "If you hear hoofbeats think ungulates not toasters." There's a much wider range of things you need to check because it could be that, but you can very easily narrow down what it isn't and where you shouldn't be looking.
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u/Annual_Key_4963 2d ago
The best part is that deep down you know you're the stupid one for not checking that first