The Shocking Pattern

This is to be placed in an area the PC's cannot escape from, such as a small room. This is written as if the PC's have entered a 40x40 room, with one door leading in.

The door to this room is heavy and reinforced. The door is not locked. The room is more or less bare. The floor appears to be covered in murky water. The water is less than 6 inches deep, is lower than the threshold of the door, but the floor is totally opaque with muddy water.

As soon as all PC's have entered, the door will slam shut, magically lock, and then the room will start to shift. The floor under the murky water will start to rise, revealing a checkerboard pattern of 10x10 squares on the floor(what PC doesn't dread the checkerboard floor? *grin*). 6 large shields will roll out from newly appeared mechanical slots in the wall and roll out onto the floor in a random pattern, a sand-timer(eg hourglass) will appear as well, from a mechanically appearing hole in the wall. A sign is revealed near the hourglass. It reads:

Place six shields on the squares in a pattern that leaves an even number of open squares in EVERY vertical AND horizontal direction.

At this point, the characters see a large sand-timer tip over. Once they read the sign, they have about 30 seconds of time to go on the sand-timer.

If the pattern doesn't match the described end condition after 30 seconds, each character gets a large shock of electricity, which stuns them into immobility, and takes about 30 seconds to wear off. The sand-timer tips over as soon as the electricity shocks the characters, they are immobile for 30 seconds, and then they have 30 seconds to place the shields before the timer runs out again, and the next shock comes. The shock damages any character in the room when it passes through.

Once the pattern is made, the hourglass tips to a neutral position and the door unlocks, the floor recedes. The PCs can take the shields if they like. If you want to include treasure, make the shields more impressive than normal, and fill the hourglass with gold dust or something.

How I ran this in actual game play:
The six shields were represented by poker chips(dice, coins, whatever is handy and small, poker chips for us). I made a 4x4 grid pattern on a white board(piece of paper will do) and placed it in the middle of the table. When the poker chips hit the white board(I tossed them liberally from behind my DM screen), the players could start working on the puzzle, and I started a countdown timer. When the timer ran out, I instructed them to look away from the puzzle, grab their character sheets and write down the damage they took from the electrical shock, and not look back at the puzzle until I had reset the timer. Then I started over, until they had solved the puzzle. They took some SERIOUS damage from this puzzle, and they are pretty smart, but it took them about 4 minutes of real puzzle solving time to come up with the solution in between all the damage rolls and such. Make sure the characters can handle the damage you're going to dish out every 30 seconds of real time.

The Answer
It's fairly obvious when the answer is right. There are actually 4 unique patterns that will solve the criteria listed. I won't actually include the answers because they are graphical in nature, and the answers become very obvious when you see them(in the "oh yeah, that's right" kind of way). What I did was I stopped the timer whenever they said they thought they had it, and told them to look away. I examined the pattern against the criteria, and if it was right, I told them, but if it was wrong, I said, nope, sorry, everyone get ready, the timer's going to start again, and then I started the timer again.

Where to use this
I personally used this in a combination puzzle. They were trying to obtain another magical item that would solve their current crisis. You could use this in the same manner, if they seek a given item, when the pattern is right, have the item mechanically appear from a hole in the wall.