The opposite could also be true. Knowing a track could =tensing up, which could cause injuries for some folks. Sometimes the best way to avoid injuries is to relax. I try to maintain a combo of bracing my legs while relaxing almost every other part of my body as much as possible for the whole ride. (really every ride)
As I have gotten older, I've personally gotten better at this.
Second, I find for many rides- though maybe SM isn't the best example- I get desensitized the more I go on all rides. If I go to Orlando for a few days, the first big ride is the one I feel the most. After a week of spinning and dropping and bouncing, each ride phases me less and less.
When it comes to SM, I suppose you could say I know the ride pretty well, but if I'm riding in the front, I can easily see what lies ahead. Even if I'm not up front, well, I find SM very easy to know. I can't see every detail, but much of the ride layout isn't hard to see. One of the biggest rough spots is near the start of the ride. While you are at the top of the ride, there's a sudden little uptick. I suspect many folks don't expect the top of the hill to have an uptick, and if you injure yourself at the start of the ride, that would make the rest of it unpleasant.
I'm sure others will say otherwise, but I notice a (small) difference between the Alpha and the Omega sides.
I also notice a difference in my reaction if I am dehydrated, especially in the high heat of a summer afternoon. With a ride like SM, I get in the AC and it feels cool, so it is easy to forget that I'm thirsty.
I'd also say, the last seat is the roughest. I can't say I have ridden SM all in a row enough times to say which is the least rough, but the last seat is definitely rougher than the rest. There is also a significant difference in the amount of leg room in the two front seats (#1 and #4). For me, I find I'm better able to brace my feet in those two front seats, especially if I'm wearing good rubber soles, but others may be better able to position themselves in the other seats.