Think payroll. I used to think payroll is relatively simple. Then I spent some time on government of Canada Phoenix pay system (go ahead... Google and weep). And it's... Insane. System has been live for a decade and still regularly gets hit with some weird scenario from some department that nobody foresaw, it wasn't captured in requirements, but upon review by business analysts is a valid scenario. Bob was a CS5 in department of defense and speaks French so gets bilingual bonus and his boss was away for half a day so Bob gets acting cs5 pay and is in public alliance union so these are the dues, and it is second Tuesday of a month and blue moon, but then Bob got moved to department of agriculture and then 3 months later realized that his previous manager at defense didn't put in his promotion on time so now you have to figure out his retro pay for when he was in defense even though everything on his file now has agriculture labour agreement and codes and rates etc etc etc. And this made up example is a fraction of the complex examples.

Clear and comprehensive Requirements are always the tricky bit, at least in business software. Twilight zone covered it perfectly and presciently decades before AI, with genies taking your requests literally and giving you unpredictable and usually negative outcomes.