Ohhh, back in the early days of client-sided inventory, boy did I have some fun trolling/griefing on servers. My crowning moment was one that took a week to set up, and six hours to pull off.
There was a server populated by douches. Talented douches, but douches nonetheless. They were a pretty strict community, very harsh to players until you were whitelisted, which often took up to a week. I persevered, if only for the fact that my friend was already with them and helped me circumnavigate most of the blocks placed on me. Soon after I was whitelisted, however, he was banned by the god-complex admin, over a pitifully petty argument, and lo were the seeds of revenge sewn.
Temple Cliff, the main building project of the server, the summation of two month's work. A sprawling city over cobblestone avenues illuminated by streetlights, majestic mansions and taverns and towers. Interweaving patterns of iron and gold lined the murals, as flowers adorned the streets. Above it all, upon its namesake cliff, sat 'The Temple', the admin's glorious monument to himself. A towering citadel set against the rising sun, its silhouette against the rising sun many a first-timer's welcoming sight to the server.
Naturally, this all had to go.
So I spent a week, week and a half maybe. I recorded who was on at what time and when, who was building what and where. When the admin was on, when the least number of the Moderator cohorts were roaming about. When the server saved, and how often. An innocent question here, a little exploring there. All the while building my own project to alleviate the inevitable suspicion that would befall me once my plan had been realised.
I found it.
At 1200 GMT, every day, the server saved its single backup. On a weekday, the players usually didn't hit until after 1600 (work & school I assumed). Heavy snowing the previous night, a snow day for my school was a guarantee. So I slept earlier that night, and woke up in the wee hours of the morning. I logged on to the server, and began the long task of casting down this digital Jericho.
I acquired TNT. And a lot of it.
Gunpowder, treason and plot. With those words I carved a labyrinthine maze beneath the city, a network of tunnels below the foundations. Below each building I carved out a massive hole, and filled each with scores of TNT. One errant click, one accidentally-set block, and the whole plan would blow up, so to speak. It was a good five hours when the final pit of TNT had been filled. Then I began connecting them. The maze became a giant fuse, a block of TNT every four spaces to ensure they didn't fly off when detonated. Another hour passed at least before I was done connecting them, and then I carved the final tunnel: the primary ignition. Far away from the city, and the Temple, it was the final piece in this plan, the 'kill-all' switch. 1130, the time till execution was still a half-hour away, so I bided my time, and watched as trickles of other players began to enter the server.
With 10 minutes to go, I ran to the Temple, and spawning some obsidian, covered the place in the stuff. I retreated back to the primary ignition hole. I needed to draw the others to the hill, too see their kingdom fall, see their arrogance that was built in iron and stone fall.
"Someone's griefed the temple, there's obsidian EVERYWHERE."
Like locusts the other players swarmed in, removing the obsidian before the wrath of the yet-to-emerge admin could befall them.
1157.
I hit the first block of TNT, and ran to the Temple.
A steady monotonous beat. *boom*....*boom*...*boom*...
Nothing. The server chat remained full of the ramblings to each other. I had failed. Somewhere the TNT had failed to detonate, was too far away. It was too risky to rebuild the tunnel, someone might notice me running off again, someone might-
"HOLY FUCK THE CITY'S EXPLODING".
Like the calm before the storm, there was but a deathly silence.
Then came the downpour.
They rushed out of the Temple, screaming disbelief until, as they reached the edge of that lofty cliff, saw their buildings fall under smoke and flying TNT. Some were in disbelief, or maybe reluctant acceptance as their buildings turned to ash, while some desperately tried to save their creations, charging off into the hellish ruins. It was all in vain. The chat was a torrent of accusations and rants and complaints, until someone mentioned that the server had a backup.
They needn't worry, they can just get the admin to revert to the previous day, their work will be safe, their precious city saved-
*SERVER BACK-UP COMPLETED*
Silenced reigned in the court of this ruined kingdom, save for a solitary "fuck".
Two months of their work, now but a pockmarked landscape. They left en masse, their rage palpable across the internet, while others, like vultures, picked through the remains for anything of value. One by one, even these scavengers logged off, never to return.
As the last one left, the sun began to set, casting its dusky glow one final time over the ruins.
Then for shits and giggles I stuck a phallus made of bedrock on the front of the Temple, and made a stream of lava come out of the tip.
TLDR: Successfully re-enacted the gunpowder plot on server full of douches