Plays, dance numbers, musical compositions and other such performance pieces usually take weeks (if not months) of practice to get it down to the precise event that the audience sees. But seeing people practice, even in a Training Montage, can be rather boring. So sometimes to catch the audience by surprise we see a performance, on stage or otherwise, that is done perfectly. This is obviously a mainstay of The Musical, as time went on finding some way to justify the music and dance numbers had to be included. The concept lies right at the Willing Suspension of Disbelief, we have to accept that a dozen people could improvise the singing, dancing and acting and somehow be completely in tune with each other. All that said, there is some Truth in Television to all of this. Jazz is a musical genre that thrives off of Improv, a skilled enough musician can intuitively adjust the key to what others were doing. Individuals familiar enough with each others dance steps can make an original performance look like weeks of practice.
- Sister Act 2 had Delores decide at the last minute to have the students change from a prim and proper rendition of "Joyful, Joyful" into a medley of rap and hip-hop styles, removing their robes and singing in their street clothes. Of course they end up winning the contest.
- Played for Laughs in The Naked Gun 2 1/2, Frank and team steal the clothes of a mariachi band in order to gain entrance to an environmental summit. When caught on stage, they end up performing a song flawlessly after what looked like an Oh, Crap! moment.
- Lampshaded in Saturday Night Live that was spoofing the dance numbers, performed by street gangs, in West Side Story. Norm MacDonald was the only one aware of what was going on.
Hello, Unknown Troper. You'll need to get known to lend a hand here.