Say for example every move has a 10% chance to succeed. Then the chance for 9 consecutive moves to succeed is 0.10^9=0.000000001 or 0.0000001%. The number can update for every step in the path, from 9 to 8 to 7, etc...
While playing, I actually couldn't tell if when the character "disobeys" then they can still accidentally choose your input direction. The behavior of that actually changes the chance. But as a developer, I would have it never choose your input if it is confused. I would show the mission percent chance of success in your UI or show nothing at all there, because it just added to my confusion in the first couple minutes of learning.