I suspect this is a common ailment among former shop mechanics. When you have an endless supply of the little things, you don't sweat them at the outset of the project. Never in my shop days did I play the mental chess of, "Ten moves from now, I will need a tandem-length brake cable for a road brake lever." I didn't have to, because I knew several of those cables were in a drawer somewhere. In the hopes of sparing other mechanics the forehead-slapping pain of poor planning (and hopefully reminding myself of some things I lack in the process), here's The Cycle's Home Shop Stockpile Shopping List:
CABLES: Since I already mentioned them, here's an obvious one. In a perfect world, I'd own the big file-box roll of 100, in road brake, mountain brake, and derailleur. A file box of tandem-length cables might be a bit much, so I'd just keep a few of those on hand. Obviously, I'd also want the file-box roll of brake cable housing and another of derailleur cable housing. Nothing makes a recabling job more fun than being able to zip out a length of housing and a fresh, shiny cable when you need it.
FERRULES AND CRIMPS: Having already belabored the point about cable ends, I'll just say get some. And while you're at it, get a bunch of cable housing ferrules. Nothing ruins the joy of that perfect recabling job like reaching the end and being one ferrule short. If you like the little rubber donuts that keep your brake cable from dinging on the top tube, get a bottle of those (I'm meh on them -- some bikes seem to really need them, while others get by just fine with naked cables).
A BIG, HONKIN' PATCH KIT: You were expecting spare tubes, weren't you? Sure, it's good to have those, since a) patching a tube during a ride sucks, and b) some flats just can't be patched. But in the luxury of my own garage, I'll bust out the sandpaper and rubber cement and make some butyl magic. In the shop, we always sold customers a new tube since the labor to patch cost more, which meant a big box of lovely, patchable tubes for the mechanics to take home and patch on their own time. I think I went five years without buying a tube.
CHAINS: If I had a nickel for every time I had to dash to the shop to buy a chain, I'd have enough nickels to nickel-plate a chain. Make sure you have a lot of whatever widget your particular chain uses to affix itself into a loop, whether that's Shimano's annoying little pins or some form of master link. And make sure the width of your widgets matches the width of your chains!
BAR TAPE: If you ride drop bars, sooner or later it will happen. Sure, you think you can unwrap the old tape with enough care to preserve it for a rewrap... and then it tears, and you're riding bare bars until you can get to the shop. In my perfect home shop, I'd have boxes of inexpensive fake cork tape (in black, since it matches everything in the fleet) just waiting on a shelf. If you ride flat bars, just find a bolt-on grip that makes you happy and buy it, cost be damned. It will pay for itself the next time you have to swap a brake lever and wreck your grip trying to get it off.
NUTS AND BOLTS: If you wrench long enough, you'll probably accumulate a decent assortment of fasteners. But without fail, if you need four, you'll have three. Or you'll want two that match for a highly-visible location and you'll have one black and one stainless. Just order a giant assortment of stainless metric hex head bolts from any number of online hardware stores and spare yourself years of hunting around the back of a drawer.
I'm sure there are plenty of other ways that a project can stall, but these are the ones that always bite me in the chamois. And in reviewing my list, I find that the shop here at The Cycle is sadly deficient in most of the items listed... so please take note when you're holiday shopping for your favorite blogger.