A friend from uni told me about the coolest intervention art project he and his friend did in Copenhagen. They created a document that looked exactly like an official document from the council, that said something like "You have left you car/house/bicycle unused for a period of time, and we therefore assume that you don't want it anymore. The Department of Public Use of Private Property (or something like that) will confiskate your car/house/bicycle in four days. If you have any objections to this, please call one of these numbers" - and then they had put the numbers to three different charity lines, where you donate money by phoning up. They placed these notes on a number of expensive cars, houses and bicycles around the city.
The funniest thing about it was that it was mentioned on the local news and they warned people of the "scammers trying to trick money out of people". They even had people from the charity organisations say that if anybody had unwillingly donated money to them, they would, of course, pay them back - "and we have absolutely nothing to do with this!" But the thing is, when you phone those charity lines they always say at the beginning what it is, and people have plenty of time to hang up if they don't want to donate. It was so clearly a joke/happening, but it was just treated like a scam. People have no sense of humour!