James Gasik
We don't talk about Pun-Pun
There's also the fall of Netheril in the Realms, when it's most powerful wizard decided to try and use magic to usurp a God, disrupting the Weave and basically undoing the very magic that held the Empire together.I agree that magocracies aren't any more (or less) likely to fail vs other types of kingdoms.
However, I think magocracies do seem to have something of a tendency to fall in spectacular fashion.
For example, the Priest King of Istar brought the Cataclysm down upon Krynn.
The Dragon Kings of Athas helped usher in the current state of affairs, of a world on its last legs.
And while it hasn't resulted in catastrophe yet, on Mystara mages in Glantri harness the power of what they call the "Radiance", the continued use of which will not only drain all magic from Mystara permanently, but could also result in an explosion that would wipe Glantri off the map!
And then the Imaskari, in response to a plague that devastated their numbers, opened portals to other worlds, bringing Orcs (well, one group of Orcs, at least) to the world, as well as the Mulhorandi and Untheric civilizations (which may even be peoples from Earth!).
Of course, there are remnants of these empires to this day, in the form of the Netherese Shades, Halruua, and the Deep Imaskari, but their mistakes changed the world forever.
The Realms is lousy with ancient empires who achieved greatness, then met folly, and of course, being a fantasy world, they tend to be magocracies, like the Elven empire that created the Mythals and Elven High Sorcery (with Cormanthor being ceded to the encroaching humans, Myth Drannor overrun by demons, etc. etc.), or the Nar, who also found out the hard way that one shouldn't play with demons.
The Realms is still kind of wacky though- Cormyr is lousy with wizards, many who work for the government, yet you still don't see common folk using magic to their benefit or spells being used to bolster the infrastructure as you do in Halruua- then again, since magic tends to freak out every time there's an edition change, I can't say I blame them for not 100% trusting it!