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_.