Harkening back to the group from the 2nd edition game I remarked on previously, here is another moment of outside the box thinking from the players. It caught me by surprise and that made it important in many ways for the players and myself.
The party was early into the chase for the thief that had stolen a very important item. They had chased the goblin into the woods and Otis had just caught up when both he and the goblin plumetted down a brush covered hole into an underground ruin. Otis had been knocked out in the fall and when he came to there was a bloody smear leading from where the goblin had been, through another small room, and out a doorway.
The rest of the party had caught up and after tending to Otis they were searching the dimly lit chambers. Among some fallen stonework Otis discovered a dagger that was missing the hilt and pommel. Picking the blade up by the rags that were wrapped around the tang he felt strange and found he could see better in the sparse lighting than a moment before. He had come into possession of an interesting dagger whose magic and drawbacks would come into play later in the campaign.
Deciding that the bloody smear was not just a ruse and that the goblin had not taken a hidden exit, the party lit a torch and started out from the door into a long hallway. They were met immediately by a volley of arrows and fell back.
Unable to see far enough down the hallway to spot their attackers they began to grow frustrated until Otis urged them to move the bright torch to the far side of the room so he could get a better look down the darkened hall. Dagger in hand, he peered from the doorway and determined that about 100 feet away was a pile of debri behind which a couple of small humanoids were crouched guarding another door.
The goblins were taking cover behind some barricades and firing shots down the long hallway keeping the characters pinned in the room and away from the door they believed the thief had escaped through. Applying their brains to the task, they took the door off the hinges and used it like a gigantic shield while moving down the hall. Then the used it to bridge up and over the barricade of junk so one of them could charge over into the enemy position.
In a few brief rounds they had defeated what otherwise would have been a dangerous combat for them and forever changed my ideas about what players were able to come up with given the opportunity to think about it.
Now when I design a dungeon environment myself I always try to include opportunities for out of the box thinking and hope players will surprise me by doing something even beyond what I had imagined.