I've been carrying cable ties around with me for ages, and finally they found a use. Poor guy at Waitrose discovered his rear wheel lock had comes loose, one like this:
One cable tie later and a thank you he was on his way. Last time I used them was three/four years ago when a ladies front brake had come off, and the nut had gone missing. Obviously the brake wouldn't work, but the cable ties got it out of the way and allowed a limp home.

One cable tie later and a thank you he was on his way. Last time I used them was three/four years ago when a ladies front brake had come off, and the nut had gone missing. Obviously the brake wouldn't work, but the cable ties got it out of the way and allowed a limp home.