Top tier comment, lol. This is the reality though, be it text, code, or design; you work hard on it, you're happy with the results, show it to others, and think it's almost done. But the 20/80 rule applies in multiple ways. You think you're at 80%, but in reality you're only at 20%. Or the last 20% will take 80% of your time.

Real life example, we were presented with a redesign of a website with an undertone of "this is it, I have worked hard on it, I am happy with how it turned out". But then we, the pesky software developers, get in and start asking questions. "Where's the mobile design? 60% of visitors visit the site through mobile but you started with a desktop design?", "What if the user increases their font size, as per the legally mandated WCAG 1.4.4 rule?", "Did you consider dark mode?" "How does this work with a touch device?" "How does this work with a screen reader?", "What about this and this and this use case?". It goes on.

I think a big difference between being a professional and a beginner is, a beginner will work very hard on the most obvious part of the problem. And might get something reasonable after a while. But the professional knows that the obvious part isn't where most of the difficulties lie, they'll probably use some tool to handle the obvious part near automatically, and they'll factor in all these invisible requirements into their original estimate.