"Fingertips" was an instrumental track on Stevie's 1962 debut studio album The Jazz Soul of Little Stevie, featuring Stevie on bongos. Promoting the album, Stevie was brought on The Motortown Revue, a concert tour packaging various Motown recording stars.
The story goes that as 12-year-old Stevie concluded "Fingertips," with him singing improvised lines and playing harmonica, at a 1963 revue performance in Chicago, he began to be escorted off the stage to thunderous cheers. The emcee said, "How about it? Let's hear it for him!" The band played his exit music. The music stopped, and musicians in the band began to change over for the next act to play, Mary Wells.
But Stevie suddenly returned to the mic, playing his harmonica again, driving the audience into a further frenzy. The surprised backing musicians (including a young Marvin Gaye on drums) struggled to join back in. Mary Wells's bass player had already replaced Stevie's, and tried to keep up, shouting, "What key? What key?"
Stevie riffed and improvised, singing his goodbyes for a few more moments, as the crowd cheered.
All this was recorded, and Motown released it as a single, with most of the song on the A-side of the record ("Fingertips, Pt. 1"), but the continuation of the song, including the chaos and encore at the end, was on the B-side.
And it was the B-side, that radio stations played, and "Fingertips, Pt. 2" became Stevie's first #1 Billboard hit. Listen for the "What key? What key?"
I just found this video, from the 1963 Motortown Revue, but from a difference performance, at what looks like The Apollo, in New York City. The same thing happens: Stevie concludes, the music stops, but he then returns to the mic for an encore.
This casts doubt on the story that he surprised the musicians with the encore in Chicago. But there's evidence of their surprise: the tentative piano notes, trying to keep up, and the "What key?" Maybe the improvised encore went so well in Chicago, it was incorporated into the act and this Apollo performance was subsequent. Regardless, it's amazing to see, as one YouTube commenter put it, the beginning of a legend.
And to further confuse matters, it can't be Marvin Gaye on drums for this performance, because he's shown as the previous act, taking bows and exiting the stage.
Here's Stevie singing "Happy Birthday," although to Martin Luther King, Jr., not himself.