I understand forward-backward (in general) pretty well and am not sure what application you're thinking of or what you mean by "a path that's impossible to happen". Anyway, yes, I agree that you shouldn't usually put 0 plausibility on views other than your current best guess.
It possible that you p=0 to go from 5:A to 6:B and the path created by forward-backward still goes from 5:A to 6:B.
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.