I usually have a contingency plan of sorts, but it's not really because I intentionally made a plan. It's more like a route map that exists out in the world, and I can see it. If I want to get from point A to point B, this is the shortest/fastest route; but if there is a road block, then there are these other less optimal routes that will ultimately get me from A to B. I don't think about everything that could possibly go wrong on my route from A to B, but if I know certain things have a high potential to happen or have a high potential to very negatively affect my route given some historical data, I will consider alternatives to work around that road block.
Ex. I may not consider a random car accident on some small side street; a) it's not likely to happen while I'm driving and b) routing around something like that is relatively easy. But If I know that there is major road construction in a certain area and that traffic tends to be really bad at certain times of day, I will consider alternate routes to avoid those areas and/or early bailout points if things go south.
The same things happens in life. For example, I had to do some survey work this past week and in the mornings the ground and dirt roads are still frozen so getting around isn't a problem, but in the afternoon it's warm enough that things are starting to thaw out so the roads turn into a big muddy mess and driving in agricultural fields gets to be a challenge. Not knowing for sure how bad the roads were going to be, Plan A was to work from the southeast corner of the project area to the northwest corner to minimize driving around time. Once the roads and cultivated fields started getting too muddy to drive in, we had to change tactics. So we jumped around the project area in the afternoon so we could stick to better roads and pastures/hay fields where there was less risk of getting stuck. The next morning, the roads were frozen again, so we went to the areas that were the most difficult to get to because of bad road access and/or having to drive across cultivated fields first, saving the areas with better access for later in the day when we knew the roads would get muddy. Before we even left the office, we knew the road conditions might be an issue, but we had no way of knowing what the exact road conditions would be until we got to the project area, so we had to plan for both possibilities (and a third possibility of being unable to drive most places and having to walk, which we didn't end up needing to worry about). Plan A was the most efficient, but ultimately didn't work out, so we moved to Plan B.
Ultimately the goal dictates the plan, not the other way around. If the goal is to get to B, then there are very few efficient routes to that goal, so I only need consider those handful of routes and plan for things along those routes. Once the goal is made, the paths (plans) are usually pretty obvious (to me anyway).