It's one of those "common wisdoms" among horse people that you should always train in private for the circumstances you will meet in public.
That's good advice. It's total nonsense, of course, but still it's good advice.
Let me 'splain by giving an example:
One time, many years ago, I rode my old mare Story up in the hills above the house where we lived then. It was an ordinary day, and an ordinary ride, on trails we used all the time. We headed up to the top of the hill on an old logging road, and I planned to ride part of the ridgetrail there and then cut back down on another trail, through the neighbor's back yard and home again.
I reckoned without the helicopter.
The land where I rode in those days was logging land, owned by
Weyerhaeuser or
Georgia-Pacific, or one of the other big lumber companies. Logging companies are, for the most part, really good neighbors. They are only on-site for a few months every ten or twenty years. They usually give permission for local hunters and hikers (and horseback riders) to use the logging roads. Even when they don't actively give permission for us to explore and build trails on their hills, logging company reps aren't around to chase us out very often, which is pretty much the same as giving permission as far as I can tell.
So in those days I was all over the hills, following logging roads and deer trails, and building trails of my own. My mare learned all the essential skills that a trail horse needs to know, including the most essential skill of all: trusting the rider.
We came up the side of a hill one day and at the very top we came upon a crop-dusting helicopter.

The contraption was parked at the top of the hill, in the middle of the trail. The engine was turned off, but the rotors were still spinning slowly, so I guessed it hadn't been parked there very long. The pilot was nowhere in sight.
Story was a little non-plussed. She had seen plenty of machines in her days on the racetrack, including trucks and tractors and starting gates and who knows what else. She'd seen plenty of stuff while travelling with me, including all the machinery associated with a working logging camp: bulldozers and boom rigs and semitrucks and chainsaws.
But she'd never seen a helicopter on the trail before.
I admit it:
I had never seen a helicopter on the trail before, either.How do you train your horse for something like that?
The answer, obviously, is that you
can't train your horse to be helicopter-safe, unless you have a helicopter handy for frequent practice sessions, which I didn't.
However, I had taken her into plenty of weird situations before we met the helicopter.
Early in our career together, I got into the habit of riding into town (about 7 miles) to the little hamburger drive-in, where I ordered a cheeseburger and fries for myself, and a really big cup of water and some sugarpackets for Story.

She was a big hit with the restaurant staff and the customers, and as a byproduct, learned to accept things like cars, traffic lights, and the sliding window at the drivein.
Another time, we were up on top of Blanchard Mountain, and as usual, I knew only vaguely where we were (but I can always find my way home, which is an important skill). I didn't realize that the big open space we had found was the launching spot for the local hang-gliders.
It wasn't exactly labeled "Hang-Glider Crossing."
But there we were, munching our lunches, when *flappity-flappity-flappity*, along came a very nice fellow and his brightly-colored wings.

I pulled a sugar packet out of my pocket, and handed it to the nice fellow to feed to my mare. She saw the packet, and forgave the fellow for his weird wings immediately. No problem.
So it was that the day Story and I met the helicopter, she wasn't worried about the machine.
Oh no.
She was hunting for the pilot.
She figured that anybody with a rig
that weird-looking was certain to have sugar packets.