Sine-wave speech -- artificially degraded speech that sounds like old Doctor Who sound effects -- can be difficult to understand but becomes clear once the listener knows what to listen for.
Listening to the sine-wave speech sound again produces a very different percept of a fully intelligible spoken sentence. This dramatic change in perception is an example of "perceptual insight" or pop-out. We have argued that this form of pop-out is an example of a top-down perceptual process produced by higher-level knowledge and expectations concerning sounds that can potentially be heard as speech.
(thx, tom)