I'm not the type of player who is tempted to run into a room and tear it apart. I really respect a well-designed room, and my problem is that I'm usually too scared to mess things up. I've never broken anything in a room, and I often fail to find things because I was too nervous to disturb a well-designed prop. When I talk about breaking a room, I'm not talking about physically breaking props, I'm talking about puzzle and lock elements that allow a team to fail because of one mistake or because of a few mistakes. Lockout safes, puzzles that rely on the original location of an item, limiting a team to one non-erasable sheet of paper, and requiring a team to select and cut a wire are all examples of game elements that allow a team to break the game.
No single action should be able to make a game unwinnable. That's really frustrating when you know that it's possible to break the game, but I've even played a few games in which we received no warning that we could potentially break the game. That's just mean, and it's not fun at all.
A lockout safe allows you to attempt a combination/key a few times, but after a wrong solution is attempted too many times, there will be a time penalty or a total lockout. This often either makes a team too scared to try a combination, adds unnecessary penalties, or causes a team to fail because of one mistake.
I recently played a game that required you to know which order you solved all the puzzles in a room in order to know which order to enter digits into a lockout safe. Fortunately we did remember which order we solved the puzzles in, but when we entered the numbers into the lock, it didn't open. Turns out, because I was a able to predict a clue that was coming, we were able to solve one puzzle earlier than the game designer wanted us to solve the puzzle. Because the solution was dependent upon the order we completed the puzzle, we had no way of knowing what the puzzle order was supposed to be. To make matters worse, when we guessed that we must have done the puzzles out of order, the game master's prewritten clues were very unhelpful in leading us to the correct order. Order of puzzle completion is a very weak form of clue giving, but it is even worse if it is possible for the players to break the puzzle by doing the puzzles in a different order.
I played another room that had props spread around the room (for secrecy we'll call them balls). The basketball was in a box. The football was on a shelf. The volleyball was in a drawer, and the soccer ball was under a blanket. The solution to the puzzle required us to match the color of the box, shelf, drawer, and blanket to the colors on a lock. The problem is that we got a ball bag in the previous area and assumed we were given the bag so that we could put all the balls in it. By the time we needed the information, we had no idea which ball was in which original location. Even when the designer provides the team with a way to reset a puzzle to its original condition, relying on original locations is just not a creative or realistic puzzle clue.
We have played a few rooms that require a team to cut a wire to disarm a bomb at the end of the room. You can do everything correctly throughout the room and fail the room by cutting the wrong wire. That is a cruel puzzle. If it is absolutely obvious to all players which wire should be cut, that's not a terrible mechanic, but if a team has completed a room, give them the satisfaction of celebrating their accomplishment. We did play one room that provided backup wires to replace an incorrectly cut wire, but that was totally unrealistic to the storyline.
The best rooms have a feeling of freedom that allows the team to explore everything, try their ideas, and move things around. Penalizing teams for this exploration removes fun from rooms.