Sometimes, despite the hours of forethought and planning you have done in planning a campaign as a DM, your players are smarter than you.
Now, before you get your pitchforks and start screaming "impossible!", hear me out.
I once pitted several of my players against an ancient wizard's laboratory, which was equal parts dungeon crawl and puzzle. In front of them stood a hallway full of deadly reflecting laser beams, bouncing back and forth across a series of runes that covered the walls. The beam originated from one focusing crystal which stood in the center of the hall.
Sure, the most agile of them could have acrobatics-ed their way through the hall no problem, but the clumsier people in heavy armor would have to either take severe amounts of fire damage as they trudged through the hallway or find another way through.
Earlier in the dungeon, I had given them two objects: a pair of gloves that were immune to fire damage, and a crystal that they found bending and redirecting a similar beam in the workshop.
In my head there were two ways through the hallway:
1) Speak the command word from the ancient wizard that would deactivate the beam.
This should be impossible, as there was no indication of what that word was and I didn't even hint at the fact that there was such a word in the first place.
2) Arm the rogue with the gloves (which I would have ruled gave a +5 on acrobatics attempts through the beam fields due to the ability to block out certain iterations of the beam without taking the damage) and have her affix the crystal to the point of origin, sending the beam harmlessly away from the reflective rune matrix.
But then they just had the wizard send the crystal floating down the hallway using "Mage Hand".
Right. Forgot about that spell.
At any rate, it was a clever solution to the puzzle, and rather than tell them "Uh, no, the crystal weighs 6 pounds and Mage Hand can only lift 5" I allowed the party to have their victory and be on their merry way.