This article is from the Bob Dylan FAQ, by Adam K. Powers and John Howells firstname.lastname@example.org with numerous contributions by others.
At the beginning of the song "Bob Dylan's 115th Dream", on the
album "Bringing It All Back Home", he starts to sing the first
verse and then suddenly stops, cracking up in uncontrollable
laughter. Why? The commonly accepted reason is that the band missed
their cue and didn't come in when they were supposed to. Because
this recording took place near the very beginning of Dylan's first
electric sessions, he wasn't used to playing with a band and just
plowed right ahead, causing everyone at the session to laugh at
the blooper. Dylan got such a kick out of producer Tom Wilson's
laugh that he insisted it remain on the album.