Pranksters et al

Oldest Prank
The most daring but successful prank in history was carried out by Doge Dandolo in 1203/4, when he first derailed, then hijacked, the Fourth Crusade, tricking the Crusaders into doing the exact opposite of what they were supposed to do. The crusade was supposed to conquer Egypt first, and then take back Jerusalem. Venice was contracted a year earlier to provide the ships to carry a sizeable army across the Mediterranean. When the crusaders arrived, their numbers were lower than anticipated, and they were unable to pay the agreed amount for the vast armada that had been assembled in Venice.
The aged Doge Dandolo, seeing an opportunity to hoodwink the crusaders, made a big show of "taking the cross" himself, and offered to forgive their debt if they would come with him on a short detour first. This was agreed, and together they first sailed to and sacked Zara, a Christian city that Venice had recently lost to Hungary, and then sailed on and sacked Constantinople, the capital of the Byzantine Empire-with Dandolo making sure that most of the crusaders never knew that Pope Innocent III was threatening them with excommunication if they did so.
There is a wonderful irony in the fact that a blind octogenarian not only managed to con the gullible crusaders out of recapturing Jerusalem for Christ, which was their primary goal, but also tricked them into sacking the main centre of Christianity in the east instead.
Finest prank
The finest prank in history was perpetrated towards the end of the second world war, against a background of gloom and horror that made it all the more brilliant. German and allied airforces were launching bombing raids on each other's factories with ferocious regularity. The Germans hatched a plan to deceive allied intelligence by building mock wooden factories painted in industrial colours, the hope being that the enemy would waste much of its precious ordinance on them. Soon enough the British figured out what the other side was up to, and sent a lone Avro Lancaster to an industrial area near Duisburg. The plane's mission: to drop a wooden bomb on one of the fake factories.
Imagine the looks on the faces of the German army officials, staring at a harmless "bomb" made from wood, and looking up at the sky, where a crew had earlier put their lives in jeopardy for the sake of a jape. Even they must have been touched by the humour of it.
The road-digging prank
The classic version, pulled by students on numerous occasions, goes as follows. A group of mischievous types procures hard hats, pneumatic drills and other construction-related items. They then cordon off a city-centre section of street and start tearing up the tarmac. One of the group calls the city police to report that students dressed as construction workers are ripping up the road-come quickly! A little while later, he calls the state police, explaining that he and his colleagues, a group of road workers only trying to do their job, are being harassed by a bunch of pranksters dressed as policemen. Before long, the city police arrive to arrest the students, and the state police come to arrest the city police. Chaos ensues.