So there I am on the home leg of my commute carrying 2 spare tubes, spanners that fit 99% of the nuts on the bikes, allen keys to fit all the right sizes, a frame pump, tire irons, and a patch kit. "Enough for any problem" I think. Then the crank falls off and the nut rolls off into the void. Of course, if I had been carrying a socket wrench and the right size socket (which I have from last bike... at home at the time) this wouldn't have been the 10km walk of shame that it became. Sigh, unless you carry a whole toolkit I guess you can never be prepared for everything.

So you were at least able to recover the crank nut? The solution is to buy two replacements and carry a spare, along with the wrenches...

Alternatively (a reminder to all of us), one could check crank bolt tension periodically. I don't do this often enough, and wait until I detect something "wrong" down there. Alas.

Funny how often I find folks stopped on the side of the MUP or road, looking in the grass for a lost crank bolt or nut. I'm sympathetic and try to help them search sometimes, but can't help. Unless we all carried portable bike shops with us that included a spare parts bin.

What happened to the crank hole cover, obviously that came off as well? Sounds like a slice of Murphy's Biking Law. That crank otherwise would not have come off in the shop with the best of tools and elbow grease with louds grunts.

Rather than carry everything, probably pre-ride checks and being "in tune" with your bike is easier to achieve. Preventative medicine, if you will.

I find that routine cleaning provides the opportunity to spot problems before they manifest out on the road. For example, while wiping grime away from crank arm and bottom bracket, I might notice a loose nut.

And what I mean by being "in tune" with your bike is paying attention to the little things, like the odd creak or slight wobble from the crank. Those things should prompt a little investigation, which may save a mid-ride breakdown.

Other little details in prep might help, too, like properly torquing bolts to spec with a torque wrench, and judicious use of light grip thread lock compound, e.g. Loctite blue 242.

+1.......I've had crank problems in the past and it's hard to think that there were no signs something was wrong early on. The prechecks are the key and in my opinion would be considered being "pre" pared.

Sometimes, it seems as though, no matter what you do, or how much you "prepare", satan relentlessly holds a firm grasp onto your number!

Hey Mr. Beanz!

Were you a Boy Scout?

No, I raised 4 active kids playing sports and that requires a while lot more prep than being a boy scout. At times I had the 6 of us out on the road for a bike ride across the city. There was only one way to be sure that one of the cranks would not fall off during a ride, equipment pre check!

Single Key Release systems! You only need an allen wrench and if the arm actually come off the bolt does not OH you said nut......

Yeah!! where was your dust cap?? shame on you for going out with your nuts exposed!!

You'd also need a whole set of spare parts.

a socket wrench and spare nut wouldn't have helped since one the crank twists off under load you've already destroyed the square taper interface +1 on just doing maintenence checks so this doesn't happen to begin with

The taper is fine. Somehow the nut got loose from the torque on the crank. I spotted it, removed the (horrible plastic) cover to check and decided that rather than walking I'd try and nurse it home gently. It was just out from the LBS because of a loose bearing in the bottom bracket. That problem is now sorted, but they must not have torqued it on tight enough. I'm tough on bikes - especially cheap ones (even if they are new) so I expect some problems from time to time. Fortunately no damage was done and its back in business. Still, the system will always find the weakest link and its not always where you expect it to be. Of course it had to be a day when I supposed to meet my kids early.

I don't even try anymore.

Around a decade ago I did the tech support for a big week long bike ride with my grandson. Flat tires, of course, are #1. I actually ran out of spare tires and had to buy some more from a local shop. Broken spokes were #2. Assorted crank and bottom bracket issues were #3. Being prepared to fix cranks and bottom brackets on the road probably means heavy spares and big tools. I'm not even going to try.

There's also a Murphy's Stationary Biking Law. A couple of months ago I had some surgery, and a friend gave me his old stationary Diamond Back to use during rehab. he's quite a bit shorter than I am, and the seat post was stuck where a 5'6" rider would use it. Adjustment was obvious, just pull the pin and slide it up...
Wouldn't move. Would NOT move. I fiddled and hammered and shoved and WD-40ed, and I couldn't get it even to wobble like it MIGHT move. Finally I gathered some tools, planning to pull off the plastic side covers to see WTF. I laid out the tools, got painfully down on the floor next to the bike (knee's still pretty tender), and, just for fun, gave the pin a two-finger tug. It slipped out with about two ounces of pressure, and the stuck seat post rocketed down, letting the saddle hit me in the head, then slam my fingers.
Works fine now.