The McGurk Effect is a perceptual illusion in which what you see affects what you hear. When what you see conflicts with what you hear, you may hear a third phoneme that is mixture of the other two phonemes. If the video shows a person saying “ga” and the audio plays “ba” you may hear a third, intermediate phoneme, “da”. This perceived intermediate phoneme is the brain’s way of satisfying the conflicting audio and visual inputs.
This illusion is known to persist even once one learns about it. While in some visual illusions one can “see through” the illusion and the illusion is broken down, one will still perceive the third phoneme if presented with conflicting audio and visual inputs.