There have been occasions that they've had them off, when a specific ride is currently a walk-on. We've had that happen on Space Mountain and Splash Mountain a couple of times. Especially with Splash Mountain, we'd want to pick up a FastPass at, say, 10:30 for perhaps 11:30, right before we'd go back to the hotel for lunch. However, the machines would be off. Sometimes 10:30 am is a bit too early to go on Splash Mountain in the winter. Brrrrrr.