The ending where you end up trapped in a cage and slowly move towards a crusher, while the second narrator cuts in and begs you to take control away from the game by exiting it, her voice growing increasingly desperate, and then is suddenly cut off by the crusher smashing into you.
Female Narrator: But listen to me. This story is not over. You can still save these two. You can stop the program before they both fail. Push escape, and press quit. There's no other way to beat this game. As long as you move forward, you'll be walking someone else's path. Stop now, and it'll be your only true choice. Whatever you do, don't let time choose for you! Don't let timeSMASH
The gradual realisation that the Countdown ending is unwinnable, the first time you play it.
The Narrator takes a sudden turn for the sadistic in that ending:
Narrator: You're only still playing instead of watching a cutscene because I want to watch you for every moment that you're powerless, to see you made humble.
The "Pawn" ending, where the Narrator restarts the story and changes it so that the monitor commanding Stanley never shuts off to give him an opportunity for escape. Thus, he is left a mindless slave as it dictates his every action, while the Narrator creepily describes how it's all hopeless and how Stanley will never escape, and then the monitor suddenly displays "Please die" and everything goes black...
The "Sneak Peek" trailer is this, especially near the end.
Narrator: Soon, you will be the one to walk. The controls will be the ones in your hands. And we'll see if your friends turn and laugh at you as they did with him. [Laughs] Oh, I think I'm going to enjoy this. When you find out how alone you truly are, I will be laughing more than anyone. Very soon, now. The end is nearly here. Let's play, everyone. Let's. All. Play.
In the HD Remake, when unplugging the telephone, the room you were in starts glitching up and deteriorating from "Narration Contradiction" after the Narrator forces you to watch a video about choices. When he redirects you back to the room with the two doors and instructs you to go left like normal, turning back through the right door leads you into a glitched dead end. After going back left, the meeting room has glitched. After going back to the room with two doors, the glitching and deteriorating further spreads until the Narrator shuts the game down and everything goes black, and then the room reappears again except dark and completely destroyed.
During the Confusion ending, the Narrator attempts to fix the breaking of the story by the use of The Stanley Parable Adventure Line™, which is supposed to lead you down the proper path, but the Line™ itself starts swerving in unexpected directions. The finale to this ending is a room with a schedule board with the description of all five false restarts so far and three additional restarts that are supposed to occur afterward, beginning with the Narrator forgetting about the previous restarts and ending with the Narrator abandoning Stanley until he dies. The Narrator objects to his memory being erased and seems to successfully abort the plotline altogether, but the game is violently restarted against his will. This is, however, a completely fresh start, but if you choose to go through the door on the right at your first choice and look through some windows in the hallway, you'll see the Adventure Line™ rambling its way around the interior of the room.
Sometimes in the HD Remix, on rare occasions, you can see another person walking through the hallway through a window. Using cheats, the Steam community have discovered that he uses the same model as Stanley; therefore, this is happening over. And over. And over. And over. And over. And over. And over. And over.
The concept of the game as a whole. Essentially, the message of the game is that not only do you have no control over anything that happens in your life, but neither does the one controlling you in the first place. If you don't hold the power, and the puppeteer controlling you doesn't hold the power, who does?