A few days ago during my morning walk, I reached for my RX100 (always in my pocket!) to photograph something I'd seen. As soon as I turned it on I could see the most dreaded message: "NO CARD".
It wasn't the first time I forgot to put the memory card back in the camera. I made that mistake once, and I learned my lesson. Since then, I keep spare memory cards almost everywhere: my wallet, my camera bags, in the car... they are cheap 8gb cards, not the fastest or highest capacity, but enough to make some images.
This little trick has saved me multiple times already, so I'd really encourage any digital photographer to do the same: buy a pack of cheap SD cards and leave them in places where you might need them.