Since this thread seems to be back on-topic, may as well contribute.
A long time ago, when clients still held the inventory, there was a server. Lovely server, spread out over a wide area. Keep in mind, this is when fire spread infinitely.
So, I prepared some underground fuses. I spent my time lighting them, then hanging out with the builders some distance away. I did this for days. They never figured out it was me. In fairness, I didn't burn down anything that couldn't be replaced, but... still, their reactions were hilarious.
Another time, there was a pretty small server focused on some fanbase or other, almost vanilla. It was unwhitelisted, but it was a small and relatively tight-knit community so there was no need for security. Some friends and I got on, and this being a small group we didn't want to ENTIRELY take advantage of their trusting ways. As such, we didn't actually destroy anything.
Instead, we explored for awhile. We found a rail system in the nether that essentially linked together several distant and decently large cities, but most of the actual people lived in either the spawn city or another specific city. One of the cities was entirely abandoned, focused mostly on a sky island only accessible via nether portal.
First, we isolated the sky island by hiding its access portal. Nobody noticed us doing this as the whole region had been long abandoned. While I kept tabs on the players, my friends snuck into their houses, disabling security systems along the way (because of course they had redstone security) and sneaking out inventories and inventories full of goods. They were giving me a tour in the meantime, and I was verbally relaying where they would be so my friends could avoid them.
We didn't destroy the goods. We didn't use them. We hid them in a set of chests under the stairs of a house on that sky island only accessible via a hidden nether portal in a city long abandoned but connected to the main rail system of the server.
When it was done, they knew something was up, of course. They didn't suspect me at all. Later that night, when they'd left, I set up a series of signs. The signs were riddles, hints leading along a chain that eventually led back to that island, and that base.
I never did follow up on it, but I can only assume they eventually figured out the chain of puzzles and recovered their stuff in full. That said, it set them back AT LEAST a day! So there!