I played in a new game this weekend. The DM was about as new to the game as I am.
We encountered a group of goblinoids near a mining camp and gave battle. Afterwards, we inspected the camp itself, which contained a tent. Someone rolled a high perception check on the tent. "There's uh, a map of the camp inside. Which you can already see," says the DM. Someone else rolls to take a closer look at the tent's contents. "Look," says the DM, "it's just a tent!" After that it was pretty much all over.
"I roll arcana to check the tent for magic!"
"I roll a histoy check to see if I know anything about the tent!"
"I attempt to use diplomacy to negotiate with the tent!"
"I climb the tent with athletics!"
And so on for a couple of minutes, as all the players displayed a rare moment of cooperative play to yank the DM's chain.
"Dude," I said to him, "you gotta just make something up. Say we find bloodstains on the tent or something."
Face it, when your player rolls a natural twenty on a perception check on that piece of scenery you hadn't put any thought into, you can't let them just go away empty handed. You have to improvise. Reward their curiosity, their initiative and their attention to detail - even if it was to the wrong detail.
I had a similar situation in the game I ran yesterday.
The players enter an area where they've been before, but recent events have transpired to turn the previously peaceful locals into hostiles. They were secretly lycanthropes, and the moon had come out!
So, the PCs roll for init and spend a few rounds sending them packing. Then they want to do what they normally do - loot the bodies and explore the area.
Well, here's the problem - they already know what was on the bodies and the area has been explored. And then someone rolls, yes, a natural twenty on their perception check.
"Okay, well, the courtyard appears much as you left it... you don't notice anything new. But, uh..."
"Uh, you detect a faint glimmering in the pool in the center. Your arcane knowledge tells you that it may have magical healing properties! +2 healing to anyone who wades in the pool!"
The PCs all rush into the pool and award themselves a couple of hitpoints. I sigh in relief. Since we're out of combat this has no effect on the encounter. And a +2 heal is hardly game-breaking anyway. But the players still feel like they got something. Bully for me!
Of course, this could still come back to haunt me. I'll have to be on guard for the moment, two sessions from now, when somebody has dropped dead and someone suddenly remembers the magical healing pool and wants to drag the body over there and see if it revives them.
Or maybe they'll want to bottle and sell the water.
This is, after all, the group that tried to burn their way through a maze of bushes and attempted to stop up a genii's magical fountain.
The point is, you should always be prepared to improvise. Your players' creativity is your friend, not your enemy. Some of the best moments in our games have come from people trying really out-of-the-box things.
As a DM, I want to get better by learning to springboard off those moments when the player tries something unheard of, rather then attempting to head them off or stuff them back in a box.