Sometimes as a Gm you will want to roll something out or present a situation whose intent is to cause an impasse. Or a situation where you know the players cannot the challenge but you need to include it for story reasons. Sometimes it's not so much a situation - its an it.
A good example of this would be when my players opened up a rift in time and space. I described something large and powerful coming through the rift. The players had no chance against it, but it's inclusion in the game was necessary for the story. I put hints and lots of foreshadowing to make this clear to the players. In fact they were so captivated with the description, that they quite literally listened as I described the creature exiting the rift. 1 of them caught on that it wasn't here to be nice and play friendly with them. He got the rest to run. My point in describing this is to show that giving hints and foreshadowing can be a useful tactic when presenting situations or in this case something, that should be avoided and not fought.
The key in doing this is to make sure that you do not present the situation in a way that frustrates the players. A sense of escape, avoidance or that they could attempt to tackle the problem should exist even if it's not true. The goal as previously discussed is to present a series of options for the players to take that end in a result that you want. At the same time preserving the players sense of freedom to choose the option that they want.
In this case I wanted them to confront the creature coming out of the rift. What I did not want them to do is to try to fight it. By using voices of people in the game, NPCs, I was able to communicate that it was unbeatable, at least to physical combat, and had to be confronted another way. This led the players to where I wanted them to go and let them think of creative ways to overcome it. Additionally I was heavy handed in foreshadowing that it was something that could only be handled through a device other than combat.
I do the exact same thing in other situations. For example the other night I was not as prepared as I normally am. In fact I had only detailed out a portion of the area the players were gaming in. To prevent the players from going to the areas I had not detailed I completely ignore the fact they were not done and instead referenced them often in game. I made them seem mundane and more the same that they had already seen. I also made sure that the general flow the game pointed away from them. When the players went towards these areas I used encounters, situations and NPCs to steer them into areas that I had detailed.
The key is to do this without clueing in your players what you doing. They have no idea what you have or have not ready. Presenting an even open front that says you are completely prepared keeps them from finding out. As a rule I lock out very few locations in my game. In fact players can go anywhere even if they shouldn't go there. I make it very clear that I do not as the game master scale encounters to their power level. Instead I put the world together and then let it run. It is full of things more powerful and less powerful than the players. Common sense and hints from the game master usually keep players from places they should not go.