Hmm, anything that's not where it should be, for example, our hack on Monday took us passed a field we regularly ride past, Monday it was the scariest place in the world, the reason, the grass had been cut and baled in black plastic! Silly mare, another one, we have a lot of HS2 stuff going on round us, first time the equipment turned up she was like 'WTF' is that! She was fine then every time we went passed until one day when it had all gone and she had a spook at the empty field.
Belle loves things to stay the same, she can and does cope with anything, even bin lorries now, but if something changes on our routes, in any of the fields or house gardens we pass it's quite scary for her.
At least the worst she does is startle and take a bit of encouragement to go past, for the most part, not counting the lamb that terrified her a few weeks ago, I swear that was trying to have her for dinner.
One thing for sure, she keeps me on my toes and makes for a few interesting rides occasionally