Mario Maker has convinced me that a lot of people genuinely suck at Mario (both from a design and gameplay perspective). On my Game of Thwomps level an overwhelming majority of people die on the first Thwomp of the level that can be avoided simply by not running full-steam into it
because why would you do that it's clearly too low for you to make it past in time. On my Dry Bones Fortress level a bunch of people have died to the first fire stick that you see coming a mile away.
My face when I look at where some people die in my levels:
No amount of smart level design can take stupidity into account.