This happened to me about 30 years ago, I'd guess I was doing 14-15mph (because I'm not that fast), and there was an OAP ahead standing at the kerb. He gave me no reason to suppose he hadn't seen me, but he stepped off the kerb at the last minute, and my left shoulder caught him square on the jaw (that he hadn't got right out in front of the bike shows how late it was when he moved).
I turned to find him spitting out a mouthful of teeth, but fortunately they were false ones, so when he assured me he was fine I rode off, but in hindsight if he'd had a brain haemhorrage an hour later I could have been in quite a difficult position (and so would he, of course).
if a car can go 30mph why can't I
It is a limit though, not a challenge, and if people spot a bike they're less likely to be expecting it to be moving at 30mph. I recall descending Kirkstone Pass at speed, and as I approached a slow bus coming up the other way trailing a queue of cars, I was quite anxious that one of them might get impatient, and think "oh, he's only on a bike, he won't be going very fast".
The key part of this is " moved backwards". Pedestrians moving backwards into collision seems to be common factor in many of the few cycle related pedestrian deaths. You cannot predict the motion of a pedestrian and if you aim to avoid, they may change direction to interception course again.
Sometimes you have to choose between braking and manouvering since you cannot do both at the same time.
Walking along the pavement toward someone coming the other way, it's very common for each to sidestep the same way, then both go the other way etc. Bike or car, I tend to be more of a braker than a swerver, but I don't really know why, it's just my automatic reaction.
