I see many people had this WA. Can someone give me a hint about it?
I think it will be useful for others.

I had WA 4 because of wrong supposition:

let probabilities are p0=1.0, p1, p2
p1 > p2
"p1" always should shoot into "p0"
"p2" always should shoot into the air

Thank you! There are really much more cases.

Another way is to type (i-1) instead of (1-i) somewhere in your code without noticing and then spend 90 minutes going over the code and try to figure out how your logic could possibly be wrong.