As seen in my sig, I killed a GM once, on Sullon Zek. I'm pretty sure it was a combination of the Sullon Zek PvP ruleset and random happenstance though:
1. Sullon Zek's ruleset was that upon character creation, you picked a diety (no agnostics allowed), and that diety hardcoded you to be on the Good team, Neutral Team, or Evil team. This made GM Jamlamin to technically be a Neutral character (and many argued that it was more than "technical", that he was the Neutral team's pocket GM).
2. GM's could be feared/charmed by mobs, or at least certain mobs, or certain levels of GM's could be (etc), because I actually saw it happen more than once during raids where GM's *happened* to show up to supervise or dole out discipline at the most inconvenient of times.
One fateful day we (Black Prophecy, the only real raiding guild on the Good team) were in basement of Ssra, killing Vyzh'dra the Cursed, when suddenly, mid fight, GM Jamlamin shows up. Great timing, as usual. Part of Vyzh'dra's encounter is that he cast either an AE charm, or an AE fear, but I think it was charm. To the collective delight of most of the server, Jamlamin happened to get charmed, and I was straight up Ric Flair animated about it (WOOO!!!), and jumped on my chance.
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Highlight of my Everquest "career", and something that I have never ever heard of any other Everquest player doing, ever, and certainly haven't seen any other screenshots of it.