My father was an engineer at Boeing, and Boeing builds in (or at least they used to) triple redundancy. Kind of hard to find a hardware store or plane mechanic mid-air over the Atlantic, so you can see why what appears to be over-building really isn't in the case of an airplane. You might think that to effect this on the ground you'd need three or more of everything, but that's not actually the case. What you need for food, for example, is obviously more than one year's supply. You aren't going to quit eating if you can help it, but what happens when your food stocks start to diminish? Grow it. That's your Plan B, put into effect as soon as you commence Plan A if you aren't already growing food. Triple redundancy enters into the equation when you stop and consider how to deal with disease, a crop failure, bad weather or a bug infestation. For example, do you know how to combine vegetable proteins in case your source of chickens or other animal protein is no longer available (quinoa happens to be a complete protein, BTW, and you can grow it in your garden in most climates)? Note that this isn't a physical 'backup' per se; knowing how to combine proteins, and that you don't necessarily have to eat strange combinations of lentils and bean sprouts at the same meal to take advantage of the benefits of combining, isn't a thing, it's knowledge you acquire. In this example, your plan A is your foodstuffs on hand, Plan B is your gardening, and Plan C, if the chickens die, is vegetarian protein combining. Part of your 'Plan C' could be also be foraging or trading skills or goods for food as well.
If you already have Plan A and Plan B, you probably already have a Plan C as well. If your Plan C is kind of vague, now's a good time to play 'what if' and think what you would do, and what you would need, if Plans A and B weren't workable for some reason. 'Plan C' isn't restricted to what you do when everything else fails, either. You can acquire and/or practice your 'Plan C' skills any time and incorporate them into your everyday life. In the process you may find that Plan C is actually better than, or at least would be a good part of, Plans A or B.
Just as you regularly check the oil level in your car, you also need to check the feasibility and practicality of your disaster contingency plans. Major changes and sometimes even minor ones should be looked at when they occur. Plan B may be to bug out, but if so, what about health issues that have arisen that were not a problem when you were planning previously, such as illness or injuries? Will you need to prepare for children who are not currently living at home but may come back to live with you? Has a parent or other relative become dependant on you for support, or has a new baby entered the picture? Plan C can also include options for alternate scenarios, such as 'Grandma comes to our house' vs. 'Grandma goes to Jane's house' and what you would do if Jane needed the supplies you put aside for Grandma or vice versa.
In my own planning, I got frustrated with all the detail, lack of detail and contradictions among different lists and lists of lists of supplies and confused as to how all those lists would actually assist me in preparing for what I personally need for emergencies. In desperation, I started thinking, instead, in terms of tasks to try to simplify things, like how would I wash dishes, or bake biscuits. When I break a task down into its composite parts or steps, I find I'm much more likely to prepare to be able to do that task without leaving out something critical.
Take washing dishes as an example. I know my process: I usually scrape off the big chunks, rinse off the soluble stuff, and then slap on some dish soap and use a scrubber to go over the surface, inside and out. Then I rinse the dish, drain it and then dry it if I'm feeling ambitious. So, I need a scraper, water, something to hold the water, a scrubber, dish soap, vinegar (for the rinse water – it removes soap and is also a disinfectant), something to put the dishes on while they drain, a towel to dry the dishes, and – what? Did I leave anything off? Yep, some way of heating the water. There's no way I'm washing dishes in ice-cold water in winter. So, add a big pot and a way of making fire to the mix just in case. Oh, and some hand lotion. No dishpan hands for me!
One step further: I'm making dirty water and towels when I wash dishes (paper plates are only good in the short term; eventually, you have to go back to the hard copies, as it were). How to dispose of the water? Easy: toss it somewhere out of the way and it will fertilize plants (I'm on acreage so that's not an issue) or filter and boil it for reuse. How to clean the towels? Back to the task-based method of breaking down what I need: water, a container, a heat source, soap, a way of drying the towels, and some way of agitating the water and soap together to get the towels clean. This will work for washing other things like clothes, so preparing for that task is already pretty much thought out. Similarly, how to make bread? We have a grill, and plenty of mesquite charcoal – might make for some interesting biscuits – but I need recipes that will work (see the Prepared Pantry web site for emergency bread recipes), a way of telling the temperature so I don't make more charcoal (oven thermometer), a way of timing the cooked items (dial timer), plus all the usual pots and measuring cups, ingredients, etc. In this instance, the only thing I needed was recipes because everything else was already on hand.
This doesn't rule out the use of lists of items. Rather, it's an adjunct to lists, especially if you are relying on someone else's list. It's just like thinking of cooking something or making something only to realize once you start in on it that you are out of a key ingredient or component, like eggs or a decent table saw blade. The recipe is a list of ingredients, and reviewing it will remind you you're out of eggs; a bill of materials can act as the 'recipe' in the same way. In the case of cooking or prototyping where you might not be relying on a recipe or plan, your 'list' is partially task based and partially list based, based on extrapolating from previous experience and your mental run-down of what you have on hand for the task as you gather materials or ingredients.
You may find that for some tasks you need to visualize what needs to be done in order to come up with a task based list if they are things you don't do every day and can't easily replicate on the spur of the moment. Try imagining how you would do a task if you didn't have all your usual tools and materials. No power = no sewing machine that runs on electricity. Do you have a treadle? Can you sew by hand? Do you have adequate scissors for cutting fabric? Needles to sew with? Thread, pins and buttons? Ever put in a zipper by hand? Sewn on a button or snap? Mentally going through the process may alert you to related things you either need to practice or learn to do from scratch as well as what you need on hand to do the task.
If you are preparing for a task that someone else usually does, you may need input from that person, keeping in mind that memory may not be reliable when coming up with a list of items needed because the person who does the task regularly may not stop and think about the process each time it's done, but rather go by rote. One way to reduce gaps in preparing is to have that person actually teach you the process; between the two of you, you should have a fairly good idea of what's needed and you can ask questions about the process to get a more in-depth view of needed supplies. Washing dishes, for example, might have additional requirements like copper cleaner for the bottoms of pots and pans or salt to clean a cast iron pot, depending on personal preference; I wash the dishes and I doubt my husband would think to have salt on hand to clean the Dutch oven.
Sometimes you have to think in terms of actual functionality to get something better than what's available for, say, agitating water while washing clothes. A personal example for me is a commercially-made device that you plunge up and down in the water. It's pricey and I read that it tends to rust. Well, toilet plungers don't rust. Problem is they have short handles. So, new toilet plunger, and a longer, screw-in handle to make it easier to use. Result: washing device that won't rust, was cheap, and is easy on the back.
Another example might be a paint brush temporarily taped to the end of a mop handle to reach a high corner when the ladder's not convenient. You can see this in action on a much larger scale on the Instructables or Lifehacker web sites, among others, where people deliberately set out to stretch the boundaries of a material's uses or find entirely new uses for something – and it's not all duct tape wallets. In reality, this is all MacGyver was doing, thinking of items not in terms of their names or usual intended purposes, but more as raw material with which to build.
A potato peeler can peel other vegetables and fruit too; a mesh tea ball can also hold bay leaves and rosemary for pot roast, and you can make a funnel out of the top of a plastic water bottle or aluminum foil or even a piece of paper in a pinch. Ignore the name of the thing – what does it do? What is it capable of being used for in terms of strength or durability or flexibility? What could you put it to use for instead of its usual or intended purpose?
If you don't have what you need to do the task, what else have you got that can you use? Everything is free game for repurposing. What you call it isn't what it's limited to. What it's usually used for isn't the only thing you can use it for.
If you've ever run out of something while cooking, you're familiar with this process. If you don't have a ½ cup measure available and need a pretty exact amount of something, what else can you use? Four measures using a coffee scoop (two tablespoons), among other things. If you run out of eggs, you can substitute unsweetened applesauce in many recipes. No nail set handy? Another nail will do in a pinch. These are perfectly good 'McGyvers', ways of achieving the desired end while using a different measure or ingredient or component.
It may take time to make these methods part of your preparations if you choose to use them; they may involve letting go of some preconceptions such as the 'proper' use for an item or you may find it too disruptive to change from a list-based to a list-and-task-based system. You may find, however, that just thinking about your planning can result in an entirely new and more effective methodology.