Audio problem: When Jerry uses the elevator, the voice of the computer reads out the floors as "twenty-third floor, fourth floor, fifth floor" etc, missing out the "twenty-" prefix until the 29th floor. The DVD subtitles list these numbers with their prefix, so this was not intentional. (00:21:10)
Audio problem: Just before Dorothy has her operation, Mike is seated at her desk looking through her medical books. Sully pays a visit to comfort her. During their moment, you can hear slight foot shuffling from the camera crew in the foreground - which is not coming from the outside. Also, you hear a slight grunting during a pause in their conversation.
Graduation - S5-E24
Audio problem: When Mr. Feeny is reading off the names of all the graduates, you can hear reverb in his voice to indicate that he is speaking into a microphone. The problem is there is no microphone in front of him. You can see Eric holding the microphone next to him and singing into it, with no audio. If that microphone is turned off, then Feeny's voice should not be amplified. If the microphone is on, then we should also hear Eric as he is singing. (00:19:38 - 00:20:56)