"I apparently know how to parallel park - I didn't know I could do that". And other stuff like it. "Oh, I could manage this project, never thought I'd manage."
The stuff in our brains not immediately accessible to our conscious thoughts. But OP didn't go there which surprised me.
Edit: and from a completely different realm, sadly - why is the baby not moving?
To the other person/parents they don't know that you know, but that's looking at knowledge of party a vs. party b, which is mot quite what all this is about. Yo your parents it's still a known unknown (hence why they ask you).
I believe the closest to unknown knowns would be temporary loss of knowledge, where at a given moment you don't manage to make the right connections between the known knowns. So it's a "could know" given the right moment or prerequisite calculation step/thought, but right there you don't so it's an unknown known.
That is, you are talking other things other entities might not know about me that I know myself which makes them, from my point of reference, known knowns. And in the event that I don't know those things myself they just become unknown unknowns.
If you don't change goalposts and keep the point of reference constant, unknown knowns wouldn't exist.
Personally, if I actually had to define unknown known from the individual entity's point of reference it would most probably refer to derived or calculated values. For example, do I know the product of 171 and 342.45673. Do I know I could calculate it? Yes. But do I know it by head? No. Which means it is eventually a known but right now it's still unknown. But even that is a stretch of a definition.
I am convinced Rumsfeld's initial proposition covers the entire space of knowledge. Maybe you could give actual examples of unknown knowns from the individual's perspective.