Dick and the Other Guy (2) - S3-E23
Audio problem: When Liam is performing karaoke at the end of the episode, his mouth is not even moving for some of the lyrics, and there aren't any vocal tracks on the song, because only the instruments are heard when he abruptly stops singing to admonish the crowd for trying to leave during his performance.
Other mistake: When Harry spits baby food on the baby (the camera lens in actuality) in retaliation for the baby spitting food on him and Tommy, Joseph Gordon-Levitt breaks character and laughs uncontrollably and looks at and gestures to off-screen crew members a couple times. (00:06:25)
Trivia: Dick is never seen walking in this episode and is only in an upright position when he moves from the chair in the lounge room to the wheelchair in the opening scene. This is because John Lithgow hurt his ankle playing tennis, and the episode was written to allow him to be in a wheelchair for the entire episode.