Anonymous prankster leaves envelopes of money in bathrooms around Japan.
No, I'm absolutely serious: Somebody in Japan has been going into men's rooms of government office buildings in Japan and is leaving envelopes of 10,000 yen bills in the stalls for people to find since April of 2007. Nobody knows who's doing it (because the bathrooms there are the only rooms that don't have securicams) or why they're doing it, but the bundles of notes are left neatly wrapped in paper with the houshuu ('remuneration') written on each of them, along with a carefully handwritten letter stating that whomever is leaving the packets of money will find the cash "useful for your persuit of knowledge."
Handwriting analysts have noted that the handwriting is getting worse and worse as more packets are left, which has lead to speculation that the individual in question is either terminally ill or elderly. Interestingly, the packets of money have been handed over to the police, who will give the packets back to the finders if no one claims them within six months.
The quote from the letter makes me wonder if this isn't an expensive reality hack: The people of Japan are said to be fascinated by this mystery, so much so that the national election has been put on the back burner. It could be said that the "persuit of knowledge" mentioned refers to trying to figure out who the anonymous, eccentric benefactor or benefactors are. It also might be that whole thing is a reality hack to get people to question what's going on around them and the money's a tool to get the attention of the people. That's what I'd do if I was going to set up a prank, anyway.