Slate has an article about the military's new "pain" gun. It targets individuals in a crowd with an invisible ray that causes a feeling similar to having your clothes on fire.
While they're promoting it now as a tool to use in countries like Iraq to target insurgents and do crowd control, I can see it being used on protestors here in the States. And I could see mass panic breaking out when a crowd thinks the "pain ray" is being used when it really isn't.
Something rather creepy about the thing - it seems part of society's march of progress in warfare and police work, trying to make something very unpleasant and messy more "clean" and scientific.