Not quite sure what the point here is. The same is true of pretty much any 'blocking' motion, there is always an 'incorrect' application. Such as if I slip to the inside of a jab or reach out to catch it instead letting it come to me. We practice our hardest to ensure that we have correct line responses, but sometimes we get it wrong... that's fighting for you!
If you're talking about using it in the way i think you mean it wouldn't work. Bong sau is not the best structure to begin with as the elbow raises which is why bong sau never stays. You don't have the same kind of support you get with tan sau or **** sau which is why it has to change quickly to something else. Besides not having a structure to back up that kind of strike it leaves you exposed to a myriad of counters because of the way you're attacking. In mantis where I've seen a smiliar strike done the person has to rely on speed and distraction, they must break their posture to strike low enough and they generally attack to the groin as that type of strike doesn't have the structure to create as much power a a straight punch or regular hooking punch. Once they do that it has no real relation to bong sau.
In combat, we will create "leak - opening". Some leaks are easy to amend. some leaks are not. Since both Tan Shou and Fu Shou have elbow down, if you add body rotation to move out of your opponent's striking path, there is not much leak there. The Bong Shou is a very special one. If your oponent's fake jab can cause your elbow to move up, it's not good.
