I don't think anyone said it had to be a tool you'd carried along with you, right?

So I've stopped at a garage a couple of times to borrow a tool. Once when I was commuting on my spare bike, and the adjustable wrench was in the saddlebag of my good bag, and a fender came loose. The mechanic gave me an odd look, but let me borrow a small wrench to tighten the mount. Didn't expect to need it, so I didn't have it!

Also had to borrow something or other on tour in Bumpass, VA. Don't remember what it was, but again, I didn't have what I didn't expect to need. That mechanic was much more personable -- after we rode 30 miles with no convenience store or gas station, he was willing and able to take a break for a chat!