For several years now, I have worked as an assistant to the Technology Coordinator at my high school. This job involves several things including system trouble shooting, network administration and running network cables throughout the high school and elementary buildings. During this experience, I have noticed a phenomenon that invariably occurs at least once per job, especially when running wire. In order to complete a job, a certain tool will be necessary. Invariably, you will not have that tool. Furthermore, the more that you need said tool, the further away it will be. This phenomenon has become so insidious that it requires naming. I call it the Law of Inverse Necessity.
The Law of Inverse Necessity can be stated in two simple sentences. If you need it, you won't have it. The more you need it, the more inaccessible it is. For example, if a certain screw must be removed in order for you to work on something, such as a computer, and, without looking at the screw, you take a flathead screwdriver, the screw will be Phillips. If the flathead screwdriver will work on the Phillips screw, then the necessary Phillips screwdriver will be just out of your reach. This is even truer with tight corners or places that are barely accessible. If you are standing on a chair in a closet and have the wrong screwdriver, the screwdriver you need will be on the floor across the room. The tighter the fit in the closet, and the more awkward the position necessary to get at the screw, the closer the screwdriver will be, but still just out of reach.
The Law of Inverse Necessity has some interesting corollaries. When you are preparing for a job, and the person with whom you are working asks if you will need a tool, then you must take it. This is the Querulous Corollary. If you get to the job without a tool that is necessary, you probably picked up that tool, then decided you wouldn't need it, and put it back down. This is the Replacement Corollary. The more time you spend looking for a tool that you think you will need, the less likely you are to actually need it. However, if you don't find it, and go to work without it, you will invariably need it. This is the Wasted Search Corollary.
After lots of experience, a person will learn how to overcome the Law of Inverse Necessity. For example, if you have worked on a particular wiring job before, and you remember what tools you wished you had, and what tools you couldn't have gotten by without, then obviously you know what tools you need to take. This experience isn't acquired overnight. Only after much experience with the Law of Inverse Necessity will the ironic nature of the law be understood.