It's the sort of mistake that it's hard to imagine a human making, is the thing. Many humans might have trouble compounding at all, but the 20 year/8 year confusion just wouldn't happen. And I think it is on the simple side of tax questions (in particular all the _rules_ involved are simple, well-defined, and involve no ambiguity or opinion; you certainly can't say that of all tax rules). Tax gets _complicated_.
This reminds me of the early days of Google, when people who knew how to phrase a query got dramatically better results than those who basically just entered what they were looking for as if asking a human.
And indeed, phrasing your prompts is important here too, but I mean more that by having a bit of an understanding of how it works and how it differs from a human, you can avoid getting sucked in by most of these gaps in its abilities, while benefitting from what it's good at. I would ask it the question about the capital gains rules (and would verify the response probably with a link I'd ask it to provide), but I definitely wouldn't expect it to correctly provide a comparison like that. (I might still ask, but would expect to have to check its work.)
