Not the hardest question, but I remember what was, in retrospect, probably the sneakiest ploy the examiner used to try to trip me up. This was for my PPL checkride. One thing my instructor had tried to get me to guard against was distractions: "Wow, look at all those police cars down there, there must be something happening," my instructor would say. He was trying to get me to look out the side window, to divert my attention from Flying The Airplane.
The examiner hit me with the same test, but he was much more subtle. We're heading back to the airport (also PWK, by the way) having pretty much completed what had been (so far) a successful ride. He starts fiddling with one of the instruments. "I wonder how we get this to work?" he asks with genuine curiosity. I look, and feel a slight surge of panic, because here's a LORAN unit in the cockpit that I'd somehow never even noticed before.
We both look at it for a few seconds-- do we push this button? that one?-- and I'm wondering how I could have spent so many hours in a cockpit and never even seen this thing before. And now I've got an examiner asking me questions about it! What do I do? Some reflex kicked in, and I said "Well, I'll have to look at that once we're on the ground." But it wasn't until several seconds later that I realized he'd been deliberately trying to distract me.
It was a good trick on his part, because it really got my general checkride nervousness to work against me. If my innate desire to Always Fly The Airplane hadn't been quite as strong, I would have locked onto this red herring that I thought was important. Darn good DPE. And darn sneaky.