You may not always be consciously considering a Plan B when you're making a decision, but that doesn't mean it doesn't exist. Queen's University researchers have discovered that the human brain's motor neurons will prepare for multiple actions before making a decision. The team learned this by conducting experiments where volunteers were asked to guide a cursor toward one of two targets before they knew which one they were supposed to pick. While it was easy for the test subjects to consciously steer down the middle, the scientists took the cursor increasingly out of sync to make people compensate through unconscious actions. Sure enough, the volunteers' movements were an average of the movement paths needed to reach the targets, not the average between the positions -- their brains were already prepping for both paths.
The findings should help understand the human mind, of course. They suggest that the brain interprets the environment as a range of potential actions, and that you're always considering options even if you only ever settle on one. However, this could also help produce smarter robots. If you could teach artificial intelligence to always have multiple solutions in mind, they could have a more human-like ability to improvise.