When people say "plan for the unexpected", they don't mean, "plan for every scenario that could occur." What they mean is, "plan on something unexpected happening." You can be sure that no matter what you are doing, you will almost never be able to control every variable. The best you can hope for is to control the ones you can anticipate, so that you can respond well to the ones you can't. In other words, if you really want to plan for the unexpected, the place to start is by planning well for the expected.
The other night I was charged with setting up the sound system at a fundraising banquet for a non-profit where I serve on the board. (New Hope Community Ministries) The sound at a banquet is key. You want people to be able to hear what is happening. They need to hear the director, the testimonies, the keynote speaker. On top of that, they need to be able to hear it over the din of background conversation and salad/pasta/dinner/desert being served. The point of a banquet is to be able to share with a large pool of donors the work that you are doing, in hopes that they will continue to support the ministry moving forward. If they can't hear what you are doing, it will have an impact on their willingness to support you. So, without making too much of it, sound is important.
I prepared in advance for every scenario I could think of and made sure that I had what I needed to make it work when we arrived at the banquet all with only an hour and a half to set up. In other words, I prepared for the expected. I made sure that I had done everything I could do in advance so that, when the unforeseen and therefore unexpected occurred, I would have time and the mental energy to respond. In this case, the unexpected was the projector being on the other side of the room from the sound system, thus creating a difficulty in getting the sound from the computer/projector to the sound board for amplification. Because I had planned of the expected situation, when the unexpected popped up, I had more than enough time and energy to make sure that the unforeseen variable was controlled by banquet time, and things went off without a hitch.
This is the same reason I manuscript all my sermons. If I prepare well for what I know; if I prepare well for the expected, it actually gives the Holy Spirit more time to show up and tell me to go in a different direction than I had planned. Because I'm confident in what has been planned, I have the confidence to deal with the unplanned in the moment.
I tell people at church the same thing. Early on in our churches life we were using an older sound system, an old projector, setting up folding chairs, ripping them down, meeting in a fellowship hall that sounded like a stereo inside a garbage can (try that sometime: stick your head in while it's playing and see what it sounds like), in a building whose A/C didn't quite work the way you hoped it would. There was a sense that in our pursuit of "excellence", error was somehow disallowed. I assured people that I had no problem if things went wrong, provided that it wasn't due to a lack of planning. If we did everything that we could in advance to make sure we were prepared for the worship service, or whatever else it was that we did, and something that we couldn't foresee and so couldn't control went wrong, that was just life. It's how things went. But if we had a hiccup in the service and it was because we didn't plan well, then that was on us. That was a failure to pursue excellence.
One time we wanted to show a testimony video for a baptism. It was really well done, and some people and put significant amount of time into putting it together. Unfortunately, the little white macbook-that-could we were using for ProPresenter didn't quite have the horsepower to push a video in full 1920x1080 HD. We had tested it for a few seconds prior to the service and it seemed to work fine, but when we ran it straight through in the service it lagged, stuttered, and ultimately would have been better with a black screen and a voiceover. Well, the first time that happened, we couldn't have anticipated that variable. But, the second time we could have. And it never happened again. Not only did we make sure that we weren't pushing full HD videos from the macbook, but we also made sure that if we were going to show a video we would watch enough of it to duplicate real world circumstances in advance to make sure it was working properly. The first time it happened was acceptable. If it happened again, it wouldn't have been.
The point is that the only way to prepare for the unexpected is to plan for the expected. Plan for the things that you know. Do the things that you can do. Do them well. And then, when the road of life takes little twists and turns, you'll have the capacity to actually handle them in the moment.