*NSYNC -- Reunited ... For About a Second

It happened ... *NSYNC JUST PERFORMED ON STAGE ... but if you blinked you might have missed it.

Justin Timberlake, Lance Bass, J.C. Chasez, Chris Kirkpatrick and Joey Fatone performed together at the VMAs for the first time since 2003 ... singing a tiny bit of "Bye, Bye, Bye" during JT's giant solo number.

Then they too went bye, bye, bye. So we gotta ask ...