So, about a week ago I decided to see if I could figure out how to take the crankset off my mountain bike (which never sees use except when my roommate uses it, which is rarely) and put it on my touring bike. Turns out that, yes, I could do so, but adjusting the front derailleur to make it work is currently beyond me, so I left it - it rides (and shifts) fine, I just can't shift down to the smallest chainring.
I haven't had time to get it working properly, a process which I think would involve swapping front derailleurs with the mountain bike as well.
Then today, sometime after one in the morning as I'm riding home from work, the left pedal felt weird. Kinda like it was twisting or wobbling at the top of every rotation. "weird", I thought, and decided to check it out once I got home. (It wasn't far at this point, maybe 2-3 kilometers, and I didn't have any tools on me) It gradually got worse, until the left crank arm fell off entirely, and I had to spend several minutes blindly sweeping my hands through the thankfully recently-mowed grass, trying to find the bolt.
Now, I don't know if it fell off because the crankset from my mountain bike is incompatible with my touring bike and can't be properly attached, or if I just failed to properly tighten it down, but if I had a toolkit on my bike I could (and would!) have stopped, found out what was wrong, and fixed it. On longer rides, I have tools in one of my bags, but I don't expect to need them on a quick trip to work.
For now though, I'm probably going to swap the cranksets back, since it's not actually doing anything to have them switched. I'll likely just take the road bike into my LBS along with the mountain bike crankset, look plaintively at someone and go "will this work?"
For the record, my touring bike is a Jamis Ventura Sport, and my mountain bike is a Schwinn....something? I'm not sure of the model. I inherited it, brand new, about four months ago, and I haven't used it much.