Japanese inmates at Abashiri prison noticed that marijuana was growing in their exercise yard, but instead of smoking it, they ratted it out. From AP Tokyo:
The marijuana plants started sprouting about a year ago, said prison official Takeshi Okamura. He said officials plucked out as many as 300 marijuana plants and treated the ground last year, but several more sprouted again this year.
Prisoners reported them to the guards.
They've been tested, but there's no evidence any of the prisoners have taken advantage of the hemp's presence.