I've had to spend a lot of time on scaffolds, and they never made me nervous unless they felt very unstable. I've stood on buckets on top of benches on top of scaffolds before. I've also walked 12" scaffold boards on stilts, walked up stairs on stilts, stood on 5 gal buckets on stilts, etc. In those situations I was reasonably nervous, but I was able to do the work.
I've been in other situations, though, where I was very scared and some where I couldn't force myself to function. I don't like being on a high scaffold with no railing. Once I went up on one that was about 16 feet high, no safety rail and nothing but a 12" scaffold board on the top. When I tried to stand up, I froze. I had to climb back down--couldn't do it.
A rail or wall close by, even if it wouldn't prevent a fall, gives a sense of security. It's a visual reference point that keeps you from losing your balance. I could never walk a scaffold board on stilts with nothing next to me, but in a stairwell, it's no problem. Brace a ladder against the steps, lean it against the far wall, run the scaffold board out to it from the landing and it's not going anywhere. As long as you don't step off the board, it's perfectly safe. A similar set up where there was no wall beside me, though, would be impossible.
My sense of invulnerability in those types of situations took a big hit after I fell six years ago. I had a 6 ft step ladder on top of
Perry scaffold (rolling scaffolds that are about 2 ft wide and 7 ft high). I needed the ladder because I had to get up in a skylight that was too small to allow another scaffold level. I had to lean the ladder against the wall, because the skylight was also too small to open it out. Only two of the four wheels had functioning wheel locks. I had locked them down, but they didn't hold. When I went up the ladder, it caused the scaffold to roll. The scaffold hit a little 3 ft knee wall and tipped. The ladder slid completely out from under me and took my feet with it. I fell about 9 ft and landed on my left arm. It shattered the wrist, and broke the humerus completely in two. I looked a bit like Buzz when he tried to fly from the stair railing and ended up on the floor with his arm lying next to him. "Here I am, and there's my arm. Gods, what a mess
it looks."
Since then, I sometimes get a sudden twinge when I'm in a shaky spot, and I'm a lot more cautious about checking out a set up before I climb up on it. What's weird is that I can often get more spooked thinking about things that I've done in the past than I got when I was actually doing them.