Cycleman's the forum expert on this sort of thing. Loosely-packed banana bags are surprisingly good at containing items with the zips undone - until they aren't (usually because of potholes). Seems to me that a variation on the banana racer design with more separate compartments (perhaps some with velcro flaps for ease of access to things like phones and snacks while seated) would be a useful innovation.
Having forgotten a couple of times, I'm fairly paranoid about whether I've done up the side pockets on my rack bags (which tend to contain important things like keys). Reaching back and groping around to check isn't always practical, so it becomes that dilemma of how sure you are that you closed it vs whether to stop...
Weirdly, I never seem to have this problem with panniers. Possibly just because I tend to use them either for utility rides from A to B without stopping, or for touring, where the one I'm likely to rummage in mid-ride is on the front rack, where I'll notice if it's not closed.
From what I pick up on group rides, it seems that the main offenders for jettisoning items while riding along are overstuffed jersey pockets and bungee cords used to secure small or amorphous items to rear racks. And clip-on rear lights, which clearly hate travelling by bicycle.