A couple of weeks ago I saw something different. When I stopped for lunch at a major tourist area I saw another cyclist locking-up. He had "odd" shoes - on the right a dark mtb shoe with a clipless pedal, but on the left he wore a regular white sneaker with a toe-clip.

Has anyone ever seen this before?

Could this work as a way of getting used to going clipless?

Never seen such before.. Maybe he's learning, as you suggested, or perhaps a pedal is in rehab.

I think the best way to get used to clipless is to just use them.
Perhaps he has a prosthetic limb and it's easier to use toe straps with it?

No, not prosthetic. He had shorts on. Everything looked normal.

If you have some types of ankle injuries the twisting action to get out of clipless pedals can be either very painful or downright impossible. So it would make perfect sense to swap the clipless pedal out for a flat one for one foot until the injury heals enough to ride. Given the choice of odd-shoed riding, and not riding at all, I know that's what I would do.