they will have the "protect oneself" instinct. someone with enough training will have the instinct and ingrained habit of protecting himself in a specific way, while thinking about what to do. which is why those who care, train their asses off so that, hopefully, doesn't happen. funnily enough, my personal experience has been exactly the contrary, i've been known to react to being startled by pulling uke waza right out of the blue, both with my hands and with my legs, especially when around other karateka, which has led to some interesting and funny scenarios, like when i was in a bus terminal with couple others after a seminar in december, and one of the guys suddenly spins and pulls off a move i think from gojushiho right next to me, and i instinctively lifted a leg to check (a habit i got after having it done to me at a competition, now i can't stop doing it) and did a soto uke, much to the amusement of the others. or the time i sneaked up on and startled the sensei i'm training with and a couple dojo mates before class with a very loud OSS! (fun times, fun times ), and fully expecting revenge i ended up doing a full force kosa uke right into nothingness when i saw the sensei bend down to tie his shoelaces. again much to the amusement of the others. i think it has to do with a couple of the drills we used to do when i trained in paraguay, one of which(which, now that i think about it, is the only one i remember ¬_¬) involved standing in shiko dachi, back against a wall, and having a dojo mate stand in front and throw punches for one to block, at increasingly shorter intervals and faster speeds. we did that pretty often, and you had to learn the ins and outs of the uke waza to do it, since natural instinct just didn't cut it there. good for learning to use double uke waza too, since with them you ingrain the reflex to always be ready to expect an attack from the opposite side to where your focusing.