While Windows RT / Windows 8 were largely panned, I think they had a lot of potential.
RT / 8 went very hard towards touch interfaces. Then 8.1 made a lot of course corrections to re-establish what's good about mouse + keyboard on Windows.
But... RT was abandoned alongside Windows Phone 7 / 8 and Windows 10 Mobile, and Windows 10 focused on mouse-keyboard once again without nearly as much thought about touch.
I really think this was one case where persistence would have paid off. A focus on "Windows everywhere" instead of (or alongside) "Microsoft apps that all exclusively push cloud services everywhere" could've put Microsoft in a position of mobile and convertible device dominance.
Editing to add, I was just reminded of the Surface Duo -- Android based. (And the announced but abandoned Surface Neo...) Another odd moment I'd describe as... "oh wait let's go back and try our old strategy but without any of the advantages of Windows app support on mobile!"