I line the bottom of the tin with slit craft foam to make some cheap and fast mini fly boxes. I have a couple of these boxes with a small assortment of proven carp flies and I keep them in my fishing packs at all times. The tins take up practically no space, are extremely light, and come in handy when you find yourself a target of opportunity. Having an emergency box with me paid off again today on my lunch break. I was walking the banks and spotted a carp rooting in very shallow water. I made a quick fly change, and two casts later was hooked up.
Although this was not a large carp by my local standards, it was a hefty handful on 3wt glass. As usual, the fly that the fish ate was black. This fish was extremely shallow, but the water was stirred up quite a bit around the fish. I am a firm believer that the black fly has a more visible outline in these conditions and gets eaten more because it's easier for them to see. Here are a few other memorable "bonus carp" that fell victim to flies from an Altoids tin.
If you find yourself fishing in warm water environments that are likely to have carp, carry a small stash of flies just in case. It's easy to find room for them, and you never know when having a good assortment of carp patterns could pay off. It might save your day, or take your outing from good to memorable.