Well, when I was still a DM on a Neverwinter Nights server (before it was closed), I was one of the most popular, but meanest and most lethal DMs out there.
We were playing in Warhammer settings, and I created a mage worshiping Tzeentch fanatically. My team destroyed him after a short battle, but turns out that Tzeentch had some bigger plans - however, he overestimated the mage and when he tried to give him more power (Mage was a level 10 Wizard, level 6 Pale Master), he lost his mind and started randomly releasing magic left and right. Any magic, you know, like what happened in Forgotten Realms when Gods were cast down from their seats. He would in one round summon a giant demon lord, while in the next he would vaporize him and turn into a pile of sludge.
What did my heroes do? They knew they couldn't come near anything like that. Simple solution - use Dominate Person on him and told him to cast all spells within 5 feet radius around him. Few rounds later, he was dead due to numerous fireballs, lightning spells and even some nastier spells cast too close to him.
Of course, I tried that trick few weeks later, with a bit stronger mage and stronger team. When the mage went berserk... my team did so, too. They rushed blindly and forced the mage to cast spells too close to him. All of the melee characters died, mage died too, but they didn't really like the death penalty...
Oh, and the time I rushed a village with an army of skeletons, who were invincible, except for one thing - water and ice attacks. So instead of wasting magic, my team just made some "buckets" out of capes and shields, lured the skeletons near a lake and defeated them by splashing with water.
This is probably one of the worst things to happen as you are DMing - over or underestimating your players. You either create a challenge that is supposedly easy, but your players die due to sheer stupidity, or they kill your Lich by simply throwing it into a fire and holding it down, so he dies within seconds...