I think misremembered sayings are the most confusing, especially to literal-minded children. My mother had an odd expression, "hard as the hobs of hell", which she often used to describe, wyrdly, a hard pound of butter straight from fridge. Now, the hobs of hell are terribly hot, I would think (the hob is the shelf at the back of a fireplace used to keep foods warm), but hard only by necessity in that they're a component that must be heat-proof.
"Not on your nelly" is another expression the history of which I know nothing, but my son, on first hearing, asked me what his nelly was...
