I agree that medications are something that can seem so innocuous but get you into hot water, particularly if some official wants to use it as an excuse to make trouble for you. So check to see what common meds you might have in your kit are actually illegal where you're travelling.

I've never heard of water filters being an issue, but I'll relate a story about flying back from Japan. Just a routine physical inspection of my check-in suitcase led to the discovery of a novelty item my wife had bought--these small mesh bags of real wood charcoal that you drop into your bath. They supposedly soak up the chlorine and other "impurities". Anyway, they were taken away since they're flammable. I hadn't thought of it, but yeah, I guess so. <shrug>

I've never tried putting activated charcoal to a match, though, Does it burn? I would think so, with its huge surface area.