A perfect prank should be funny, but it should also make a statement and actually make the world a better place. I figure the prank recently revealed to have been perpetrated by the Untergunther, a French "cultural guerrilla" group, comes pretty close to fulfilling that definition. Members of the Untergunther secretly repaired a clock in the Pantheon, a Paris landmark. The clock had been broken for decades -- until one day it was mysteriously working again. The Guardian reports
For a year from September 2005, under the nose of the Panthéon's unsuspecting security officials, a group of intrepid "illegal restorers" set up a secret workshop and lounge in a cavity under the building's famous dome. Under the supervision of group member Jean-Baptiste Viot, a professional clockmaker, they pieced apart and repaired the antique clock that had been left to rust in the building since the 1960s. Only when their clandestine revamp of the elaborate timepiece had been completed did they reveal themselves.
Instead of thanking them, the Centre of National Monuments, which felt embarrassed by the prank, decided to bring legal action against the group. The court recently cleared the charges against them.