The horse I lease is quite bombproof - we've encountered deer, big birds, ATVs, dirt bikes, cars, abandoned RVs in the woods, bridges, water, etc. He was once standing in the barn doorway and a piece of plywood fell from the hay loft and hit him (lightly, but it was still a big surprise and scared me!) - he stepped sideways one step and just stood there.
The one thing that'll cause him to spook a bit is when another horse spooks. He's still very good and doesn't take off, but he will either freeze or sidestep for one or two steps. Last weekend another horse in front of us spooked at some deer (taking off into the woods thankfully not far) and he sidestepped two steps and froze. He also didn't like the look of an overturned boat we ran across in the woods once. After we walked progressively nearer and nearer to it a a few times he got over the fear. It's wonderful to have such a brave horse for trail riding.