Watch this man saying some syllables. What he says, and what it looks like he is saying, sometimes do not match. You will perceive a sound that ought to be produced somewhere in between the two; for example, if his lips produce /b/, but he pronounces /g/, you ought to perceive /d/.
To help understand this illusion, you should first turn down the sound and watch his lips for the sounds he appears to produce. Then turn up the sound and listen to what he says (i.e., without looking). Finally, watch and listen at the same time. You will notice that you no longer perceive some of the sounds the same way you did with vision only.
Click on the left-hand button underneath the image to start the recording (Video clip courtesy of Dr. Dominic W. Massaro. For more information, see Perceiving Talking Faces: From Speech Perception to a Behavioral Principle, MIT Press, 1997).