Here's mine

Hm. I can think of a bunch of them, actually.

In the early days of Ultima Online, there was a rather hilarious incident where I went to the only town in the game that had no guards—and was consequently a haven for player killers who guards would murder on sight—and dropped a complete suit of the best armor in the game (forged by my grandmaster smith) on the ground before running behind a building and turning myself invisible. All of the pieces were snatched up in seconds by the unscrupulous dickheads who frequent the place, and they promptly started killing one another in an attempt to collect the complete set. And I imagine some joined in just because it was an excuse to kill people. When it was down to just a few people (and the rest hadn't had time to resurrect and return) I came out of hiding, along with three friends, slaughtered the survivors, looted everything from everyone, and portaled out.

After I got tired of the direction that game was going, I tried out Asheron's Call for a few months. In one of the game's updates, a town that had been a hub for trading was blown up. Most of the mules (characters dedicated to holding excess loot) were stationed in a tower, so with that tower gone, mules who logged in appeared high in the air and fell to their deaths. A friend and I watched the carnage for hours, laughing the whole time. When it started to taper off a little, we even started scoring them, along with at least a dozen other people.

I have a ton of stories from Starsiege: Tribes, but I think the best one was about how my little clique found a clipping bug that allowed us to destroy nearly everything in an enemy base on the most popular map without ever even going inside. It required perfect placement (one particular pixel, with no margin for error) of an explosive charge, and no one else ever managed to replicate the technique. It drove people bonkers for months.

In Counter-Strike, which I was never any good at, my favorite moment was when I was out in the open and got flashbanged. I started firing my desert eagle wildly as I ran for cover. When the white faded from my screen, I found that I had killed two people at extreme long range with headshots. I was immediately accused of cheating and banned from the server. Totally worth it.

Eventually, I got tired of online gaming and started to focus more on single player games, and the earliest I can recall that really stood out was from Vampire: The Masquerade – Bloodlines. The hotel mission. If you know it, you probably know exactly why it was the first game that ever managed to creep me right the hell out.

Man, I could keep talking about this stuff for hours, but I'll stop there.