You think you have everything planned out well: the Glabrezu is disguised as one of the party's friendly NPCs, and is working at earning their trust. Some of the party members are suspicious, though, and tensions are high. Inevitably he will be discovered and cinematically reveal his true large, pincer-demoned form, sow chaos amongst their ranks by casting Confusion, and deliver a mocking one-liner before teleporting away to safety.
Then, in the one round of combat, the witch hits the demon with a spell that makes it provoke attacks of opportunity from all 6 of the characters and NPCs surrounding it if it fails its will save. It rolls a one.
6 lucky attacks and one extremely lucky crit later: Dead demon.
In another game, you make a puzzle room involving a large, demonic dog rooting around in the bones of a tomb. You need to recover some of these bones to resurrect a story-relevant Prince, but attacking the dog head-on will probably involve a long and drawn-out battle. Clever players might think of ways of distracting it just long enough for someone to sneak down into the tomb and grab the bones that they need. Or the wizard could use his scroll of "Banishment" and send the dog back to hell.
How about the vampire-wizard with the information that you need to track down an ancient, floating citadel?He resides in a manor in the middle of the forest, and is a truly formidable opponent. After a few rounds of parley, it becomes clear that he doesn't intend to allow the party to leave... alive. This fight is most certainly going to be awesome, what with the large collection of spells, resistances, and spell-like abilities at his disposal! After all, he's a wizard AND a vampire! Oh wait, the party wizard has Disintegrate too.
I'll just, uh, wipe the dust off of his spellbook and other powerful magical items for you.
So what have we learned here? Well, for one, spells can be massively over-powered if used correctly, but so can a lucky power attack vorpal scythe crit. More often than not, your players will fondly look back on those brief combats as not trivial, but epic. "Remember that time I one-shotted the vampire wizard? That was awesome!" While it is important to occasionally instill fear in the hearts of your players, don't forget that it is also good for them to occasionally feel like Heroes. Beowulf wouldn't be Beowulf if he wasn't able to tear Grendel's arm off with his bare hands. Even Smaug was defeated by one extremely well-placed arrow. In a world of epic fantasy and heroic deeds, every so often it's important for the planets to align and for your players to critically succeed.