A very prevalent misunderstanding/error though - 'comparing fractions of different things' in general.

i.e. here the basic issue is that it's really more like two different potatoes, each with different water content but the same amount of flesh; they're different potatoes so obviously they have different masses, and that's the denominator unit of the (water content ratio) fraction in a sense.

It rolls off the tongue ever so nicely though.

Consider: The Potato Conundrum is just not as fun to say or easily memorable (IMHO).

I tend to agree, we need another word for 'tricky or misleading problem'.