It's not uncommon to see people hop on servers just to mess with people, but it's truly a dilemma when they show up, and within 3 minutes they have all Diamond armor and a Diamond sword, and they're senselessly attacking people, and then flying away to escape their name being seen.

Vanilla servers are extremely susceptible to this problem, because they have almost no defense against common hacks/griefing clients. Many people familiar with Java are more than happy to poke around in the minecraft.jar and fiddle with it to create workarounds to allow flying, noclipping and ore-sighting, fastplacing, super-mining and so on, and then posting the mods up for download.

The mods will find their way into the hands of some pretty annoying people, who just like to bug everyone for the heck of it. No one really likes these people, at least, no one with a brain stem anyway.

Solution: I think people developing these mods should keep their work to themselves and their friends. Don't post them on Rapidshare, where immature 11 year olds can download them. (No offense to the younger group of Minecraft players)

Also-any public server should be using Bukkit, and the NoCheat plugin. It's being developed by a guy who is aware of this reality, and puts in a huge effort to prevent hackers and griefers from ever having the upper-hand. Any Vanilla servers should be backing up frequently, preferably daily.