First of all, everyone should check of Equine Guelph's Lameness Lab! It's very cool.
I've had Scarface since he was three, making it a total of about 8.5 years (he is officially 12 now) and in that time we've definitely gotten to know each other fairly well. He still surprises me sometimes (like how much he loves being my niece's walk-trot horse or when he is abnormally calm), but all-in-all, I usually know what to expect.
My dad and I trailered over to the neighbour's arena the night before New Year's to go for a little ride (Friday night). Another family was riding as well, so altogether there were 4 riders and 1 man lunging a young horse (I think it was a 2009 baby). The arena owner was sitting in the viewing area at the end of the arena watching. I was warming Scarface up and just working on getting him back into shape after his time off. I didn't notice, but while I was at the far end of the arena, the owner let in a dog and was playing with it quietly in the viewing area. Apparently Scarface didn't notice either because when as we came cantering down the long wall toward the viewing area, he saw the dog that wasn't there before and came to a dead stop. He stared at it for a minute and then I thought he was fine and asked him forward again. Instead, he started motoring backwards at high speed, convinced that there was some kind of monster in the viewing area. I let him look for another moment, then asked him forward again, but he was having no part of it. By this time, everyone was watching us and the man lunging (also one of the dog's owners) was asking if everything was ok and called the dog over so Scarface could see it and see that it was in fact a dog and not a monster. That helped and a bit later Scarface didn't mind going to that end of the arena.
But the moral of the story is that everyone made a bit of a big deal about it and I was kind of embarrassed. Everyone at home and at and barns I've boarded at have known Scarface well enough to not be the least surprised when he spooks, but I don't know the people around this arena that well, so they aren't used to it yet, even though I'm sure I've told everyone how big of a chicken Scarface is.
Do your horses have any weird quirks that other people don't quite know how to react to?