The first time: used MCCheat, an old cheating program from 2010, to cover a furry server's spawn in soul sand, soon after it was added to the game. Not particularly destructive, though I filled every room of every house in the spawn's city with it in such a way that the soul sand seemed to be exploding out of them and spilling out onto the streets.
The second time: soon after the whole Doridian-installed-a-backdoor-in-his-mod-and-abused-it fiasco, I found his server and griefed the hell out of it. Can't remember exactly how, but it involved a lot of manually destroying blocks + lava for hours on end without ever being caught.
The third time: fast forward to 2013 and a server called 4craft, a server made for all of 4chan's boards to participate in, with their own factions and islands. I was in /vg/'s faction and had gotten everyone geared up with either full diamond or full enchanted diamond by mining hundreds of diamonds - using a client with an x-ray mod in it - and making a mob grinder for our faction to use. Eventually, while exploring, I managed to find /v/'s island. They lived on the top/rim of the /v/olcano; their base was built over a giant lake of lava. After climbing the mountain by myself and reaching the outskirts of their base, I immediately noticed two things. 1: I was outnumbered 20 to 1. 2: There weren't any trees around for hundreds of blocks in any direction, except for their tree farm on top of the /v/olcano, with about 50 large oak trees in it, which they hadn't /f claimed. That had to go.
I mentioned about 20 people being nearby, but they either didn't see my name tag, (The closest people were about 30-40 blocks away, and name tags are hard to spot when there are blocks between you and the player), or they did, but assumed I was just some faction member hanging out at the tree farm. Either way, for the next hour, I began meticulously cutting down the trees. Since this was only intended to be an exploratory mission, I hadn't planned on doing this, so I didn't bring an axe. I did it by hand at first, eventually switching to using a hastily crafted stone axe. (I had been carrying a stack or two of cobblestone, in case I needed it to climb things or anything like that, and they came in handy as scaffolding while slowly crouch-walking from tree to tree, about 5-10 blocks off the ground on a self-made cobblestone walkway.)
This took around an hour. I had a flint and tinder on me, (Can't remember why; as a defensive weapon to use while running away? Who knows), and once most of the trees and leaves were down, I finished the job by lighting the few remaining ones on fire, and continuing to destroy them as fast as I could. Perfect timing: one of the 20 or so /v/ members finally exited their base and came toward the tree farm. At first, he was walking with an axe in his hand, but he spotted me, stopped for a moment, then switched to a diamond enchanted sword and began sprinting. I only just /f homed in the nick of time; on that server, you couldn't use teleportation commands if enemies were within around 30 blocks of you. I told my faction members what happened, how /v/ now needed a new source of trees and we celebrated.
Shortly after, we decided to raid the /v/olcano, now that we knew its location. (I wrote down the coords once I found it.) There were 4 of us - me and 1 guy in full enchanted diamond, the other 2 in full, regular diamond. We sailed from our island to theirs, then made the several hundred block trek to the base of the /v/olcano. After climbing up to the top, we all began crouching, as planned, and laughed about how they still hadn't rebuilt their only tree farm yet; it seemed like they hadn't thought to store any saplings in their chests. Well, we slowly crouchwalked over to the very edge of their faction protections, which didn't quite cover their whole base. Using an x-ray texture pack, I decided on the room we'd enter - a roughly 15x7x5 room whose walls were covered with large chests. There was a single player in there with full diamond and an unenchanted diamond sword. This would be a slaughter. Of course, there were now around 15-20 other players nearby in adjacent rooms, but none of them saw us yet, roughly 50% of them had no armor at all, roughly 35% had only full iron or worse and only about 15% had unenchanted full diamond. Meanwhile, our gear aside, we'd even thought to bring things like strength potions and fire resistance potions. We could take 'em all.
We dropped in on the guy by mining through the ceiling. The moment I was through, the others came in with me; I took on the 1 guy with my full enchanted diamond, sword included, while the other 3 split up and went into different rooms. The moment I started hitting the guy, who now knew we were there, due to hearing us breaking blocks/mining a tunnel over the ceiling as we got closer, I knew something was wrong. He wasn't flashing red, he wasn't being knocked back, and my chat was being spammed with a message while I let him have it. I finally glanced down to read it after a second. "You cannot hurt [his name] on his own territory!"
It was a slaughter alright, but not the kind we were hoping for. Realizing we were all about to die, I switched from trying to kill an invincible player to using up all the lava buckets I'd had the foresight to place in my hotbar, to make the room a mess and defend myself with lava. Naturally, I voiced my complaints about this to the server owner, and he said he changed that particular setting, so it would never happen again. However, I too am a server owner, and I've tested this with more than 5 different versions of factions. In all of them made since that year, it doesn't matter if you change all those settings to allow players who aren't /f enemies to hurt each other; no matter how you change the settings, no one can ever hurt faction players on their own territory, if they aren't your /f enemy. It's bullshit, but the creators of the factions plugin refuse to do anything about it.
So, keep that in mind before you try raiding anyone on a factions server. You don't wanna end up like the four of us.