It was a trip to Italy in 1985 that changed something I carry ALL the time now:
Our group (my wife had won the trip for her performance as a store manager and, along with the other 75 or so top managers in the company, and their spouses, we were a large gang) had had a day at the Vatican and one woman had taken a spill. I didn't see it happen, but heard the whole story later. It seems she tripped and fell forward, striking her head/face on cobblestones, cutting her face up and destroying her glasses.
At dinner that night, she showed up with bandages and the ugliest eyeglass frames possible, along with a story: It seems that the Vatican guards attended to her and took her to a Roman clinic for treatment of her injuries. Then, they took her to an optician's shop -- one of the few open on a Saturday afternoon.
There, no one spoke English and she didn't speak much Italian, but she did have her glasses prescription with her. Since such prescriptions are all numbers, and apparently international, they were able to replace her lenses. The frames, well, they were, at best, serviceable.
Since that trip, I have always carried a copy of my eyeglasses prescription in my wallet -- every day and everywhere. I also keep the last pair of glasses in a hard case in my bag, even if there's been a major change in my prescription. With uncorrected 20/400 vision, I have little choice.