http://pierroule.com/ZappaRealBook/TheRFZBook.htm

Songs written with one idea in mind have been known to mutate into something completely different if I hear an 'optional vocal inflection' during rehearsal. I'll hear a 'hint' of something (often a mistake) and pursue it to its most absurd extreme.

The 'technical expression' we use in the band to describe this process is: "PUTTING THE EYEBROWS ON IT." This usually refers to vocal parts, although you can put the eyebrows on just about anything.